ast: Fix module path resolution for sibling modules in root

[Harishankar14]

Currently, the compiler fails to resolve module paths when a file in the root directory (e.g. 'bar.rs') attempts to load a sibling module (e.g. 'foo.rs') if a 'bar/' directory does not exist. The compiler incorrectly assumes that if 'bar.rs' is not 'mod.rs', it must look exclusively in a subdirectory.

This patch adds a fallback mechanism in 'Module::process_file_path'. If the subdirectory search fails, it strips the implicit subdirectory and attempts to resolve the module in the parent directory, consistent with Rust 2018 path rules.

Fixes #4402

gcc/rust/ChangeLog:

* ast/rust-ast.cc (Module::process_file_path): Add fallback search for sibling modules when subdirectory search fails.

gcc/testsuite/ChangeLog:

* rust/compile/issue-4402.rs: New test.
* rust/compile/issue_4402_foo.rs: New test.

Thank you for making Rust GCC better!

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