Relax regex re. recognizing requirements.txt

Zhihan found that the plotly project has some requirements files that
are not recognized by FawltyDeps, with names like requirements_abc.txt.

Using the underscore as a separator in the filename does not match our
regex, which requires a word boundary ("\b") around the "requirements"
string in the filename.

Fix our regex: There is no reason why the underscore separator should
disqualify this as a requirements file, and simplifying the regex to
".*requirements.*\.(txt|in)" is unlikely to introduce false positives
(files that should _not_ be recognized as requirements.txt files, but
now are).

Also add unit tests precisely for parsing filenames of dependency files
as I could not find another test case that tested exactly this.

fawltydeps/extract_declared_dependencies.py

@@ -367,7 +367,7 @@ def first_applicable_parser(path: Path) -> Optional[ParserChoice]:
         lambda path: path.name == "pyproject.toml", parse_pyproject_toml
     ),
     ParserChoice.REQUIREMENTS_TXT: ParsingStrategy(
-        lambda path: re.compile(r".*\brequirements\b.*\.(txt|in)").match(path.name)
+        lambda path: re.compile(r".*requirements.*\.(txt|in)").match(path.name)
         is not None,
         parse_requirements_txt,
     ),

tests/test_deps_parser_determination.py

@@ -2,10 +2,12 @@
 
 import logging
 import shutil
+from pathlib import Path
 
 import pytest
 
 from fawltydeps.extract_declared_dependencies import (
+    first_applicable_parser,
     parse_source,
     parse_sources,
     validate_deps_source,
@@ -16,6 +18,45 @@
 
 from .utils import assert_unordered_equivalence, collect_dep_names
 
+
+@pytest.mark.parametrize(
+    ["path", "expect_choice"],
+    [
+        pytest.param(Path(path), expect_choice, id=path)
+        for path, expect_choice in [
+            ("requirements.txt", ParserChoice.REQUIREMENTS_TXT),
+            ("setup.py", ParserChoice.SETUP_PY),
+            ("setup.cfg", ParserChoice.SETUP_CFG),
+            ("pyproject.toml", ParserChoice.PYPROJECT_TOML),
+            ("anything_else", None),
+            ("sub/requirements.txt", ParserChoice.REQUIREMENTS_TXT),
+            ("sub/setup.py", ParserChoice.SETUP_PY),
+            ("sub/setup.cfg", ParserChoice.SETUP_CFG),
+            ("sub/pyproject.toml", ParserChoice.PYPROJECT_TOML),
+            ("sub/anything_else", None),
+            ("/abs/requirements.txt", ParserChoice.REQUIREMENTS_TXT),
+            ("/abs/setup.py", ParserChoice.SETUP_PY),
+            ("/abs/setup.cfg", ParserChoice.SETUP_CFG),
+            ("/abs/pyproject.toml", ParserChoice.PYPROJECT_TOML),
+            ("/abs/anything_else", None),
+            ("requirements.txt/wat", None),
+            ("setup.py/wat", None),
+            ("setup.cfg/wat", None),
+            ("pyproject.toml/wat", None),
+            ("requirements-dev.txt", ParserChoice.REQUIREMENTS_TXT),
+            ("test-requirements.txt", ParserChoice.REQUIREMENTS_TXT),
+            ("extra-requirements-dev.txt", ParserChoice.REQUIREMENTS_TXT),
+            ("abc_requirements.txt", ParserChoice.REQUIREMENTS_TXT),
+            ("requirements_abc.txt", ParserChoice.REQUIREMENTS_TXT),
+            ("more_requirements_stuff.txt", ParserChoice.REQUIREMENTS_TXT),
+            ("evenrequirementsthis.txt", ParserChoice.REQUIREMENTS_TXT),
+        ]
+    ],
+)
+def test_first_applicable_parser(path, expect_choice):
+    assert first_applicable_parser(path) is expect_choice
+
+
 PARSER_CHOICE_FILE_NAME_MATCH_GRID = {
     ParserChoice.REQUIREMENTS_TXT: "requirements.txt",
     ParserChoice.SETUP_PY: "setup.py",