Revert "Allow NU_LIBS_DIR and friends to be const" (#8501)

Reverts nushell/nushell#8310

In anticipation that we may want to revert this PR. I'm starting the
process because of this issue.

This stopped working
```
let-env NU_LIB_DIRS = [
    ($nu.config-path | path dirname | path join 'scripts')
    'C:\Users\username\source\repos\forks\nu_scripts'
    ($nu.config-path | path dirname)
]
```
You have to do this now instead.
```
const NU_LIB_DIRS = [
    'C:\Users\username\AppData\Roaming\nushell\scripts'
    'C:\Users\username\source\repos\forks\nu_scripts'
    'C:\Users\username\AppData\Roaming\nushell'
]
```

In talking with @kubouch, he was saying that the `let-env` version
should keep working. Hopefully it's a small change.

crates/nu-engine/src/call_ext.rs

@@ -39,7 +39,7 @@ pub trait CallExt {
         &self,
         engine_state: &EngineState,
         stack: &mut Stack,
-        name: &str,
+        pos: usize,
     ) -> Result<T, ShellError>;
 }
 
@@ -111,9 +111,9 @@ impl CallExt for Call {
         &self,
         engine_state: &EngineState,
         stack: &mut Stack,
-        name: &str,
+        pos: usize,
     ) -> Result<T, ShellError> {
-        if let Some(expr) = self.get_parser_info(name) {
+        if let Some(expr) = self.parser_info_nth(pos) {
             let result = eval_expression(engine_state, stack, expr)?;
             FromValue::from_value(&result)
         } else if self.parser_info.is_empty() {

crates/nu-engine/src/env.rs

@@ -1,9 +1,9 @@
 use std::collections::HashMap;
 use std::path::{Path, PathBuf};
 
-use nu_protocol::ast::{Call, Expr, PathMember};
+use nu_protocol::ast::PathMember;
 use nu_protocol::engine::{EngineState, Stack};
-use nu_protocol::{PipelineData, ShellError, Span, Value, VarId};
+use nu_protocol::{PipelineData, ShellError, Span, Value};
 
 use nu_path::canonicalize_with;
 
@@ -18,6 +18,8 @@ const ENV_PATH_NAME: &str = "PATH";
 
 const ENV_CONVERSIONS: &str = "ENV_CONVERSIONS";
 
+static LIB_DIRS_ENV: &str = "NU_LIB_DIRS";
+
 enum ConversionResult {
     Ok(Value),
     ConversionError(ShellError), // Failure during the conversion itself
@@ -222,17 +224,6 @@ pub fn path_str(
     env_to_string(pathname, &pathval, engine_state, stack)
 }
 
-pub const DIR_VAR_PARSER_INFO: &str = "dirs_var";
-pub fn get_dirs_var_from_call(call: &Call) -> Option<VarId> {
-    call.get_parser_info(DIR_VAR_PARSER_INFO).and_then(|x| {
-        if let Expr::Var(id) = x.expr {
-            Some(id)
-        } else {
-            None
-        }
-    })
-}
-
 /// This helper function is used to find files during eval
 ///
 /// First, the actual current working directory is selected as
@@ -248,7 +239,6 @@ pub fn find_in_dirs_env(
     filename: &str,
     engine_state: &EngineState,
     stack: &Stack,
-    dirs_var: Option<VarId>,
 ) -> Result<Option<PathBuf>, ShellError> {
     // Choose whether to use file-relative or PWD-relative path
     let cwd = if let Some(pwd) = stack.get_env_var(engine_state, "FILE_PWD") {
@@ -272,32 +262,36 @@ pub fn find_in_dirs_env(
         current_dir_str(engine_state, stack)?
     };
 
-    Ok((|| -> Option<PathBuf> {
-        if let Ok(p) = canonicalize_with(filename, &cwd) {
-            return Some(p);
-        }
+    if let Ok(p) = canonicalize_with(filename, &cwd) {
+        Ok(Some(p))
+    } else {
         let path = Path::new(filename);
-        if !path.is_relative() {
-            return None;
+
+        if path.is_relative() {
+            if let Some(lib_dirs) = stack.get_env_var(engine_state, LIB_DIRS_ENV) {
+                if let Ok(dirs) = lib_dirs.as_list() {
+                    for lib_dir in dirs {
+                        if let Ok(dir) = lib_dir.as_path() {
+                            // make sure the dir is absolute path
+                            if let Ok(dir_abs) = canonicalize_with(dir, &cwd) {
+                                if let Ok(path) = canonicalize_with(filename, dir_abs) {
+                                    return Ok(Some(path));
+                                }
+                            }
+                        }
+                    }
+
+                    Ok(None)
+                } else {
+                    Ok(None)
+                }
+            } else {
+                Ok(None)
+            }
+        } else {
+            Ok(None)
         }
-
-        let lib_dirs = dirs_var.and_then(|dirs_var| engine_state.find_constant(dirs_var, &[]));
-        // TODO: remove (see #8310)
-        let lib_dirs_fallback = stack.get_env_var(engine_state, "NU_LIB_DIRS");
-        let lib_dirs = lib_dirs.or(lib_dirs_fallback.as_ref())?;
-
-        lib_dirs
-            .as_list()
-            .ok()?
-            .iter()
-            .map(|lib_dir| -> Option<PathBuf> {
-                let dir = lib_dir.as_path().ok()?;
-                let dir_abs = canonicalize_with(dir, &cwd).ok()?;
-                canonicalize_with(filename, dir_abs).ok()
-            })
-            .find(Option::is_some)
-            .flatten()
-    })())
+    }
 }
 
 fn get_converted_value(

crates/nu-engine/src/eval.rs

@@ -828,7 +828,7 @@ pub fn eval_element_with_input(
                             ],
                             redirect_stdout: false,
                             redirect_stderr: false,
-                            parser_info: HashMap::new(),
+                            parser_info: vec![],
                         },
                         input,
                     )
@@ -899,7 +899,7 @@ pub fn eval_element_with_input(
                             ],
                             redirect_stdout: false,
                             redirect_stderr: false,
-                            parser_info: HashMap::new(),
+                            parser_info: vec![],
                         },
                         input,
                     )