Closes #11689.

Closes #11689.

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

Closes issue #13961. Thanks to Charles-François Natali for the patch.

It seems better to cache the finder for the cwd under its full path
insetad of '' in case the cwd changes. Otherwise FileFinder needs to
dynamically change itself based on whether it is given '' instead of
caching a finder for every change to the cwd.

This is to bring it more in line with what PEP 328 set out to do with
removing ambiguous absolute/relative import semantics.

The C accelerator was not always imported for cElementTree's tests. (there's still an issue with --huntrleaks switch)

Closes #13995.

Closes #13995.

an __import__ implementation that takes care of basics in C and punts
to importlib for more complicated code.

Patch by Tshepang Lekhonkhobe.