Fix module resolution for relative subdirectory paths

[pedjaradenkovic]

Problem

Running the CLI with a relative path that contains 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).

Absolute paths and bare filenames work; only relative subdirectory paths break. This is a regression on main.

Root cause

The top module is opened via open_from(template_dirs, module_name + ".plain"), and get_template_directories always sets template_dirs[0] = dirname(filename). So module_name must be the bare stem.

bc6563b ("prevent stripping module subdirectory") changed the derivation to keep the relative subdirectory prefix for non-absolute paths:

module_name = stem if is_absolute() else with_suffix("").as_posix()

For a relative invocation this double-joins with template_dirs[0]:

• template_dirs[0] = examples/example_hello_world_python
• module_name = examples/example_hello_world_python/hello_world_python
• join → examples/example_hello_world_python/examples/example_hello_world_python/hello_world_python.plain → not found

Since sibling import/requires modules must resolve relative to template_dirs[0], keeping a subdir in module_name is fundamentally incompatible with that design. Reverting to always-stem is the correct fix.

Fix

• plain_file.py: module_name = plain_source_file_path.stem (always).
• tests/test_plainfileparser.py: test_relative_subdirectory_path_resolves replicates the CLI's path decomposition (full relative path + its dir as template_dirs[0]). Verified it fails on the old code and passes on the fix.

Verification

• Full suite: 216 passed.
• Relative subfolder CLI invocation now works (both dry-run and parse).
• Subfolder requires confirmed working end-to-end: top module in a subfolder with a sibling requires, and requires: [nested/leaf] in a deeper subfolder. Negative control (requires: [nested/DOES_NOT_EXIST]) errors on exactly that path, proving nested resolution is active.

Behavior note

Top-module build/conformance folders are keyed by the stem again (e.g. build/main) — the pre-bc6563b behavior. This reverts the subdir-nested build path introduced by bc6563b (which had no test and broke the primary CLI path).