Fix circular source causing Nushell to crash (#12262)

# Description

EngineState now tracks the script currently running, instead of the
parent directory of the script. This also provides an easy way to expose
the current running script to the user (Issue #12195).

Similarly, StateWorkingSet now tracks scripts instead of directories.
`parsed_module_files` and `currently_parsed_pwd` are merged into one
variable, `scripts`, which acts like a stack for tracking the current
running script (which is on the top of the stack).

Circular import check is added for `source` operations, in addition to
module import. A simple testcase is added for circular source.

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# User-Facing Changes
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It shouldn't have any user facing changes.

crates/nu-std/src/lib.rs

@@ -3,7 +3,7 @@ use nu_engine::{env::current_dir, eval_block};
 use nu_parser::parse;
 use nu_protocol::{
     debugger::WithoutDebug,
-    engine::{Stack, StateWorkingSet, VirtualPath},
+    engine::{FileStack, Stack, StateWorkingSet, VirtualPath},
     report_error, PipelineData,
 };
 use std::path::PathBuf;
@@ -50,10 +50,9 @@ pub fn load_standard_library(
         }
 
         let std_dir = std_dir.to_string_lossy().to_string();
-        let source = format!(
-            r#"
+        let source = r#"
 # Define the `std` module
-module {std_dir}
+module std
 
 # Prelude
 use std dirs [
@@ -65,14 +64,14 @@ use std dirs [
     dexit
 ]
 use std pwd
-"#
-        );
+"#;
 
         let _ = working_set.add_virtual_path(std_dir, VirtualPath::Dir(std_virt_paths));
 
-        // Change the currently parsed directory
-        let prev_currently_parsed_cwd = working_set.currently_parsed_cwd.clone();
-        working_set.currently_parsed_cwd = Some(PathBuf::from(NU_STDLIB_VIRTUAL_DIR));
+        // Add a placeholder file to the stack of files being evaluated.
+        // The name of this file doesn't matter; it's only there to set the current working directory to NU_STDLIB_VIRTUAL_DIR.
+        let placeholder = PathBuf::from(NU_STDLIB_VIRTUAL_DIR).join("loading stdlib");
+        working_set.files = FileStack::with_file(placeholder);
 
         let block = parse(
             &mut working_set,
@@ -81,13 +80,13 @@ use std pwd
             false,
         );
 
+        // Remove the placeholder file from the stack of files being evaluated.
+        working_set.files.pop();
+
         if let Some(err) = working_set.parse_errors.first() {
             report_error(&working_set, err);
         }
 
-        // Restore the currently parsed directory back
-        working_set.currently_parsed_cwd = prev_currently_parsed_cwd;
-
         (block, working_set.render())
     };