Fix module resolution for subdirectory paths (top and required modules)

[pedjaradenkovic]

Problem

Two related failures when a .plain module lives in / references a subdirectory:

1. Running the CLI with a relative path containing a subdirectory fails:

   $ python plain2code.py examples/example_hello_world_python/hello_world_python.plain
   Error: Module does not exist (examples/example_hello_world_python/hello_world_python).
   

2. A top module that requires a module in a subdirectory (requires: [dir_name/b]), rendered from outside its folder, fails when the PlainModule tree is built:

   Error: Module does not exist (b).
   

Root cause

get_template_directories always folds dirname(filename) into template_dirs[0]. But the full top-module filename was still passed to plain_file_parser, so the directory was counted twice when opening the file (open_from joins template_dirs[0] + the module name).

• Required/imported modules are already passed as template-relative names (dir_name/b.plain), so they must keep their subdirectory prefix to resolve.
• The top module must drop its directory prefix (it's already in template_dirs).

An earlier attempt fixed only the top module by forcing the bare stem — but that stripped the subdirectory from required modules too, breaking case 2.

Fix

• plain2code.py: normalize the top module to os.path.basename(args.filename) at the call sites (its directory is already in template_dirs). This is the frame the parser expects.
• plain_file.py: plain_file_parser preserves the relative subdirectory prefix in module_name (so dir_name/b resolves and is retained in build/conformance paths); absolute paths fall back to the stem.

With this, both the top module and required modules in subdirectories resolve correctly, and required-module subdirectory structure is preserved under the build folder (build/dir_name/b).

Tests

tests/test_plainfileparser.py (fixtures in tests/data/requires_subdir/):

• test_parser_keeps_subdirectory_in_module_name — parser keeps nested/leaf for a template-relative name.
• test_requires_module_in_subdirectory_resolves — builds the full PlainModule tree with a required subdirectory module (the path that broke) and asserts module names + build folder (build/nested/leaf).

Verification

• Full suite: 217 passed; black / flake8 / mypy clean.
• Original relative-subfolder CLI invocation works.
• requires: [dir_name/b] from outside the folder now builds the module tree and reaches the API instead of failing at "Module does not exist".