Resolve modules in subdirectories via caller-side basename

The top module's directory is already folded into template_dirs by
get_template_directories, so passing the full filename to plain_file_parser
double-counted the directory (relative subdirectory paths failed with
'Module does not exist'). Normalize the top module to its basename at the
call sites in plain2code.py; plain_file_parser keeps preserving relative
subdirectory prefixes so required/imported modules (e.g. 'dir_name/b') resolve.

Add PlainModule-tree and parser-level regression tests.

Author: pedjaradenkovic

plain2code.py

@@ -355,7 +355,7 @@ def main():  # noqa: C901
     # Parse the plain file (and its required modules) once; reused by dry-run and rendering.
     try:
         plain_module = plain_modules.PlainModule(
-            args.filename,
+            os.path.basename(args.filename),
             args.build_folder,
             args.conformance_tests_folder,
             template_dirs,

tests/data/requires_subdir/main_in_subdir.plain

@@ -0,0 +1,12 @@
+---
+requires:
+  - nested/leaf
+---
+
+***implementation reqs***
+
+- Main implementation requirement.
+
+***functional specs***
+
+- Display "hello, world"

tests/data/requires_subdir/nested/leaf.plain

@@ -0,0 +1,7 @@
+***implementation reqs***
+
+- Base implementation requirement for the leaf.
+
+***functional specs***
+
+- A base functionality from the leaf.

tests/test_plainfileparser.py

@@ -27,6 +27,32 @@ def test_regular_plain_source(get_test_data_path):
     }
 
 
+def test_parser_keeps_subdirectory_in_module_name(get_test_data_path):
+    # A template-relative filename with a subdirectory (as passed for required/imported modules)
+    # must keep its subdirectory prefix in the module name so it resolves under template_dirs.
+    data_dir = get_test_data_path("data/requires_subdir")
+
+    module_name, _, required = plain_file.plain_file_parser("nested/leaf.plain", [data_dir])
+
+    assert module_name == "nested/leaf"
+    assert required == []
+
+
+def test_requires_module_in_subdirectory_resolves(get_test_data_path):
+    # Regression: rendering a top module (referenced by basename, its dir supplied via
+    # template_dirs) that `requires` a module in a subdirectory must build the full PlainModule
+    # tree without a "Module does not exist" error, preserving the subdirectory in build paths.
+    import plain_modules
+
+    data_dir = get_test_data_path("data/requires_subdir")
+
+    module = plain_modules.PlainModule("main_in_subdir.plain", "build", "conformance", [data_dir])
+
+    assert module.module_name == "main_in_subdir"
+    assert [m.module_name for m in module.required_modules] == ["nested/leaf"]
+    assert [m.module_build_folder for m in module.required_modules] == [os.path.join("build", "nested/leaf")]
+
+
 def test_unknown_section():
     plain_source = """
 ***definitions***