Fix AST parsing issues with function scope, generics, and test packages

[fank]

This PR fixes several parsing issues that were causing errors when analyzing large codebases:

Issues Fixed

1. Function-scoped declarations

Problem: Function-local variables were being tracked as file-level declarations, causing "duplicate declaration" errors when the same variable name was used in multiple functions.

Solution: Modified the visitor to return nil after emitting FuncDecl nodes, preventing descent into function bodies. Import cycle detection only needs package-level declarations.

2. File emission order

Problem: All files were emitted when processing a Package node, but ImportSpec nodes were processed later during the AST walk. This caused all imports to be assigned to the last file in the package, leading to "duplicate import" errors.

Solution: Added package context tracking (currentPackage, currentDirName) to the visitor. File nodes are now emitted individually as they're encountered, with proper filename lookup from the package context.

3. Blank identifier

Problem: The blank identifier _ was being tracked like a regular declaration, but it's used for compile-time checks and can appear multiple times.

Solution: Skip tracking declarations named _ in the addGenDecl function.

4. External test packages

Problem: Packages ending in _test (external test packages) were being processed, but they can coexist with regular packages in the same directory and cannot be imported.

Solution: Added filtering in main.go to skip packages with names ending in _test.

5. Generic type receivers

Problem: Methods on generic types like *Foo[T] or Foo[T, U] were causing errors because the receiver name extraction didn't handle generic type syntax.

Solution: Added extractTypeName function that recursively extracts base type names from ast.IndexExpr and ast.IndexListExpr nodes. Updated receiver type handling to support generic types.

6. Error messages

Enhancement: Added file paths to all error messages for easier debugging.

Testing

These fixes were tested on a large codebase with:

• Generic types with single and multiple type parameters
• Extensive test coverage with external test packages
• Multiple functions using the same local variable names
• Compile-time interface checks using blank identifiers

The tool now successfully generates import cycle graphs for such codebases.

[samlitowitz]

2. File emission order

Problem: All files were emitted when processing a Package node, but ImportSpec nodes were processed later during the AST walk. This caused all imports to be assigned to the last file in the package, leading to "duplicate import" errors.

Solution: Added package context tracking (currentPackage, currentDirName) to the visitor. File nodes are now emitted individually as they're encountered, with proper filename lookup from the package context.

Resolved by #14

[samlitowitz]

Some of these issues have been addressed and some are not issues.