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-parser/src/parser.rs

@@ -7,6 +7,7 @@ use crate::{
     ParseError, Token, TokenContents,
 };
 
+use nu_engine::DIR_VAR_PARSER_INFO;
 use nu_protocol::{
     ast::{
         Argument, Assignment, Bits, Block, Boolean, Call, CellPath, Comparison, Expr, Expression,
@@ -19,10 +20,10 @@ use nu_protocol::{
 };
 
 use crate::parse_keywords::{
-    is_unaliasable_parser_keyword, parse_alias, parse_def, parse_def_predecl,
+    find_dirs_var, is_unaliasable_parser_keyword, parse_alias, parse_def, parse_def_predecl,
     parse_export_in_block, parse_extern, parse_for, parse_hide, parse_keyword, parse_let_or_const,
     parse_module, parse_old_alias, parse_overlay_hide, parse_overlay_new, parse_overlay_use,
-    parse_source, parse_use, parse_where, parse_where_expr,
+    parse_source, parse_use, parse_where, parse_where_expr, LIB_DIRS_VAR,
 };
 
 use itertools::Itertools;
@@ -781,6 +782,25 @@ pub struct ParsedInternalCall {
     pub error: Option<ParseError>,
 }
 
+fn attach_parser_info_builtin(working_set: &StateWorkingSet, name: &str, call: &mut Call) {
+    match name {
+        "use" | "overlay use" | "source-env" | "nu-check" => {
+            if let Some(var_id) = find_dirs_var(working_set, LIB_DIRS_VAR) {
+                call.set_parser_info(
+                    DIR_VAR_PARSER_INFO.to_owned(),
+                    Expression {
+                        expr: Expr::Var(var_id),
+                        span: call.head,
+                        ty: Type::Any,
+                        custom_completion: None,
+                    },
+                );
+            }
+        }
+        _ => {}
+    }
+}
+
 pub fn parse_internal_call(
     working_set: &mut StateWorkingSet,
     command_span: Span,
@@ -800,6 +820,10 @@ pub fn parse_internal_call(
     let signature = decl.signature();
     let output = signature.output_type.clone();
 
+    if decl.is_builtin() {
+        attach_parser_info_builtin(working_set, decl.name(), &mut call);
+    }
+
     // The index into the positional parameter in the definition
     let mut positional_idx = 0;
 
@@ -5312,7 +5336,7 @@ pub fn parse_expression(
                 arguments,
                 redirect_stdout: true,
                 redirect_stderr: false,
-                parser_info: vec![],
+                parser_info: HashMap::new(),
             }));
 
             (
@@ -6158,7 +6182,7 @@ fn wrap_expr_with_collect(working_set: &mut StateWorkingSet, expr: &Expression)
                 decl_id,
                 redirect_stdout: true,
                 redirect_stderr: false,
-                parser_info: vec![],
+                parser_info: HashMap::new(),
             })),
             span,
             ty: Type::String,