case-sensitive filesystems -- which is not the default case. Along the way also
fixed the skipping of tests when sys.dont_write_bytecode is true.

Closes issue #5442 again.

that is in fact the case.

Closes issue #5442.

importlib.test.support to importlib.test.source.util.

case-insensitive file system, leading to test failures. This was due to using
the TestCase objects directly instead of the guard in the test_main() function.
Move over to a class decorator instead to control if the tests should be run.

planned for the package.

There are no docs yet, but they are coming once the API for the first new
function, importlib.import_module() is finalized.