Allow NU_LIBS_DIR and friends to be const (#8310)

# Description

Allow NU_LIBS_DIR and friends to be const they can be updated within the
same parse pass. This will allow us to remove having multiple config
files eventually.

Small implementation detail: I've changed `call.parser_info` to a
hashmap with string keys, so the information can have names rather than
indices, and we don't have to worry too much about the order in which we
put things into it.

Closes https://github.com/nushell/nushell/issues/8422

# User-Facing Changes

In a single file, users can now do stuff like
```
const NU_LIBS_DIR = ['/some/path/here']
source script.nu
```
and the source statement will use the value of NU_LIBS_DIR declared the
line before.

Currently, if there is no `NU_LIBS_DIR` const, then we fallback to using
the value of the `NU_LIBS_DIR` env-var, so there are no breaking changes
(unless someone named a const NU_LIBS_DIR for some reason).


![2023-03-04-014103_hyprshot](https://user-images.githubusercontent.com/13265529/222885263-135cdd0d-7884-438b-b2ed-c3979fa44463.png)

# Tests + Formatting

~~TODO: write tests~~ Done

# After Submitting

~~TODO: update docs~~ Will do when we update default_env.nu/merge
default_env.nu into default_config.nu.

crates/nu-command/src/deprecated/source.rs

@@ -45,7 +45,7 @@ impl Command for Source {
     ) -> Result<PipelineData, ShellError> {
         // Note: this hidden positional is the block_id that corresponded to the 0th position
         // it is put here by the parser
-        let block_id: i64 = call.req_parser_info(engine_state, stack, 0)?;
+        let block_id: i64 = call.req_parser_info(engine_state, stack, "block_id")?;
 
         let block = engine_state.get_block(block_id as usize).clone();
         eval_block_with_early_return(

crates/nu-command/src/env/source_env.rs

@@ -1,6 +1,8 @@
 use std::path::PathBuf;
 
-use nu_engine::{eval_block_with_early_return, find_in_dirs_env, redirect_env, CallExt};
+use nu_engine::{
+    eval_block_with_early_return, find_in_dirs_env, get_dirs_var_from_call, redirect_env, CallExt,
+};
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
@@ -42,12 +44,15 @@ impl Command for SourceEnv {
 
         // Note: this hidden positional is the block_id that corresponded to the 0th position
         // it is put here by the parser
-        let block_id: i64 = call.req_parser_info(engine_state, caller_stack, 0)?;
+        let block_id: i64 = call.req_parser_info(engine_state, caller_stack, "block_id")?;
 
         // Set the currently evaluated directory (file-relative PWD)
-        let mut parent = if let Some(path) =
-            find_in_dirs_env(&source_filename.item, engine_state, caller_stack)?
-        {
+        let mut parent = if let Some(path) = find_in_dirs_env(
+            &source_filename.item,
+            engine_state,
+            caller_stack,
+            get_dirs_var_from_call(call),
+        )? {
             PathBuf::from(&path)
         } else {
             return Err(ShellError::FileNotFound(source_filename.span));

crates/nu-command/src/system/nu_check.rs

@@ -1,4 +1,4 @@
-use nu_engine::{find_in_dirs_env, CallExt};
+use nu_engine::{find_in_dirs_env, get_dirs_var_from_call, CallExt};
 use nu_parser::{parse, parse_module_block, unescape_unquote_string};
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack, StateWorkingSet};
@@ -106,7 +106,12 @@ impl Command for NuCheck {
             _ => {
                 if let Some(path_str) = path {
                     // look up the path as relative to FILE_PWD or inside NU_LIB_DIRS (same process as source-env)
-                    let path = match find_in_dirs_env(&path_str.item, engine_state, stack) {
+                    let path = match find_in_dirs_env(
+                        &path_str.item,
+                        engine_state,
+                        stack,
+                        get_dirs_var_from_call(call),
+                    ) {
                         Ok(path) => {
                             if let Some(path) = path {
                                 path