[perlinm]

Motivation

These are small leftovers from making the coverage check run qLDPC's compiled functions as plain Python (#836 to #840). The rule for the plain_search_setup fixture of the distance tests was broader than its reason, so some tests gave up their runs with compiled set-up code without needing to. The test that keeps compiled-on-request functions in qldpc.codes.distance scanned decorators in the source, so it missed a function registered by a call. The distance README restated rules and listed files that are defined elsewhere. The library also used both "ordinary Python" and "plain Python" for the same thing.

Changes

• The plain_search_setup docstring now says that a test needs the fixture if it relies on a compiled-on-request function calling a function that the test replaces, since compiled callers would not call the replacement. Tests that replace a function that only plain-Python code calls no longer request the fixture, so they run with compiled set-up code again. These are the coset oracle test in coset_leader_search_test, the overlap test in brouwer_zimmermann_test, and the table-steps test in scan_test.
• _compilation_test.py checks the location of compiled-on-request functions against the runtime registry, in a child process that imports every module, so it also catches f = compiled_on_request(f). The import loop is shared with the existing selection oracle, and each of the code strings has its own comment.
• The distance README no longer names the files that use @compiled_on_request, and points to the fixture's docstring rather than restating its rule.
• "Ordinary Python" becomes "plain Python" in _compilation.py and the docstring of compile_search_setup.

Testing

The tests that no longer request the fixture pass with compiled and with plain set-up code. The location test fails, naming the module, when a function in _binary.py is registered by a call.