Rename main to script name when running scripts (#9948)

# Description

This PR does three related changes:

* Keeps the originally declared name in help outputs.
* Updates the name of the commands called `main` in the user script to
the name of the script.
* Fixes the source of signature information in multiple places. This
allows scripts to have more complete help output.

Combined, the above allow the user to see the script name in the help
output of scripts, like so:


![image](https://github.com/nushell/nushell/assets/547158/741d192c-0a39-45a7-8f36-3a0dc8eeae2b)

NOTE: You still declare and call the definition `main`, so from inside
the script `main` is still the correct name. But multiple folks agreed
that seeing `main` in the script help was confusing, so this PR changes
that.

# User-Facing Changes

One potential minor breaking change is that module renames will be shown
as their originally defined name rather than the renamed name. I believe
this to be a better default.

# Tests + Formatting
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crates/nu-protocol/src/engine/engine_state.rs

@@ -783,16 +783,22 @@ impl EngineState {
         decls.into_iter()
     }
 
+    #[allow(clippy::borrowed_box)]
+    pub fn get_signature(&self, decl: &Box<dyn Command>) -> Signature {
+        if let Some(block_id) = decl.get_block_id() {
+            *self.blocks[block_id].signature.clone()
+        } else {
+            decl.signature()
+        }
+    }
+
     /// Get signatures of all commands within scope.
     pub fn get_signatures(&self, include_hidden: bool) -> Vec<Signature> {
         self.get_decls_sorted(include_hidden)
-            .map(|(name_bytes, id)| {
+            .map(|(_, id)| {
                 let decl = self.get_decl(id);
-                // the reason to create the name this way is because the command could be renamed
-                // during module imports but the signature still contains the old name
-                let name = String::from_utf8_lossy(&name_bytes).to_string();
 
-                (*decl).signature().update_from_command(name, decl.borrow())
+                self.get_signature(decl).update_from_command(decl.borrow())
             })
             .collect()
     }
@@ -807,13 +813,10 @@ impl EngineState {
         include_hidden: bool,
     ) -> Vec<(Signature, Vec<Example>, bool, bool, bool)> {
         self.get_decls_sorted(include_hidden)
-            .map(|(name_bytes, id)| {
+            .map(|(_, id)| {
                 let decl = self.get_decl(id);
-                // the reason to create the name this way is because the command could be renamed
-                // during module imports but the signature still contains the old name
-                let name = String::from_utf8_lossy(&name_bytes).to_string();
 
-                let signature = (*decl).signature().update_from_command(name, decl.borrow());
+                let signature = self.get_signature(decl).update_from_command(decl.borrow());
 
                 (
                     signature,

crates/nu-protocol/src/plugin_signature.rs

@@ -48,8 +48,8 @@ impl PluginSignature {
     }
 
     /// Update signature's fields from a Command trait implementation
-    pub fn update_from_command(mut self, name: String, command: &dyn Command) -> PluginSignature {
-        self.sig = self.sig.update_from_command(name, command);
+    pub fn update_from_command(mut self, command: &dyn Command) -> PluginSignature {
+        self.sig = self.sig.update_from_command(command);
         self
     }
 

crates/nu-protocol/src/signature.rs

@@ -237,8 +237,7 @@ impl Signature {
     }
 
     /// Update signature's fields from a Command trait implementation
-    pub fn update_from_command(mut self, name: String, command: &dyn Command) -> Signature {
-        self.name = name;
+    pub fn update_from_command(mut self, command: &dyn Command) -> Signature {
         self.search_terms = command
             .search_terms()
             .into_iter()