Issue #14043: Speed up importlib's _FileFinder by at least 8x, and add a new importlib.invalidate_caches() function.

importlib is now often faster than imp.find_module() at finding modules.

Also wrap two looong lines.

Closes issue #14054.

upon malformed POST request.

upon malformed POST request.

upon malformed POST request.

to to makes bytes mutable, before deciding to add bytearrays.)

Closes #11689.

Closes #11689.

Optimize importlib's case-sensitivity check by wasting as little time as possible under case-sensitive OSs.