feat: improve error message about missing identifier when the cause i…

…s a missing import statement

When the missing identifier matches a module name, a note is added to the error message highlighting the fact that the import statement is missing.
Example:

```text
            r#"error: unknown identifier `test_proto2`
   ╭─[line:4:5]
   │
 4 │     test_proto2.foo
   │     ─────┬─────
   │          ╰─────── this identifier has not been declared
   │
   │ Note: there is a module named `test_proto2`, but the `import "test_proto2"` statement is missing
───╯
```

yara-x/src/compiler/errors.rs

@@ -130,10 +130,12 @@ pub enum CompileErrorInfo {
 
     #[error("unknown identifier `{identifier}`")]
     #[label("this identifier has not been declared", span)]
+    #[note(note)]
     UnknownIdentifier {
         detailed_report: String,
         identifier: String,
         span: Span,
+        note: Option<String>,
     },
 
     #[error("unknown module `{identifier}`")]

yara-x/src/compiler/ir/ast2ir.rs

@@ -19,6 +19,7 @@ use crate::compiler::ir::{
 use crate::compiler::{
     CompileContext, CompileError, CompileErrorInfo, PatternId,
 };
+use crate::modules::BUILTIN_MODULES;
 use crate::re;
 use crate::re::parser::Error;
 use crate::symbols::{Symbol, SymbolKind, SymbolLookup, SymbolTable};
@@ -342,6 +343,19 @@ pub(in crate::compiler) fn expr_from_ast(
                         ctx.report_builder,
                         ident.name.to_string(),
                         ident.span(),
+                        // Add a note about the missing import statement if
+                        // the unknown identifier is a module name.
+                        if current_struct.is_none()
+                            && BUILTIN_MODULES.contains_key(ident.name)
+                        {
+                            Some(format!(
+                                "there is a module named `{}`, but the `import \"{}\"` statement is missing",
+                                ident.name,
+                                ident.name
+                            ))
+                        } else {
+                            None
+                        },
                     ),
                 ));
             }

yara-x/src/compiler/tests/errors.rs

@@ -535,6 +535,27 @@ rule test {
    │     ────────┬───────  
    │             ╰───────── this identifier has not been declared
 ───╯
+"#,
+        ),
+        ////////////////////////////////////////////////////////////
+        #[cfg(feature = "test_proto2-module")]
+        (
+            line!(),
+            r#"
+rule test {
+  condition:
+    test_proto2.foo
+}
+        "#,
+            r#"error: unknown identifier `test_proto2`
+   ╭─[line:4:5]
+   │
+ 4 │     test_proto2.foo
+   │     ─────┬─────  
+   │          ╰─────── this identifier has not been declared
+   │ 
+   │ Note: there is a module named `test_proto2`, but the `import "test_proto2"` statement is missing
+───╯
 "#,
         ),
         ////////////////////////////////////////////////////////////