Don't read scripts without extensions as modules in namespace mode

[getim]

The FindModuleCache currently matches files without an extension when --namespace_packages is enabled while the docs don't mention that this should be the case. The "near-miss" logic collects candidates for modules, which could correctly include a directory foo/bar/baz when looking for foo/bar/baz. However, the current logic also picks up a file foo/bar/baz. This means that, if both a file foo/bar/baz and foo/bar/baz.py exist, the first one is actually picked, resulting in unexpected behaviour.

The condition that checks exists_case on foo/bar/baz should also check that it is indeed a directory by checking that it is not a file. I'm open to different fixes of course, but this seemed like the most obvious and least impactful change to make.

This PR modifies 2 tests:

• add test-data/packages/modulefinder/pkg1/a to verify that ModuleFinderSuite.test__no_namespace_packages__find_a_in_pkg1 is indeed working correctly even without the patch because it's not running in namespace mode.
• add test-data/packages/modulefinder/nsx-pkg3/nsx/c/c, making ModuleFinderSuite.test__find_nsx_c_c_in_pkg3 fail, which the patch fixes.

To give one real-world example of this scenario: Bazel's Python rules construct a wrapper-script with the same name as the main Python-file without the extension for a py_binary-target. If some other Python rule depends on this //foo/bar:baz py_binary-target, it sees both foo/bar/baz and foo/bar/baz.py in the same directory, incorrectly picking up the wrapper-script instead of the module. Dependencies on a py_binary might be a bit of an edge-case, but Python execution of these targets does pick up the right file, so Mypy should probably as well.

[hauntsaninja]

Thanks, this looks good!

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉