Fix some comments and variable names.
Execute rmtree_errorhandler in an except block to test whether
it reraises an exception.

This requires keyring and any backends to be installed separately.
Once discovered, it will be used to retrieve credentials by index URL
and netloc before prompting. If the user is prompted and the
credentials work, they will (optionally) be saved to keyring against
the netloc of the requested URL.

utils.encoding.auto_decode() was broken when decoding Big Endian BOM
byte-strings on Little Endian or vice versa.

The TestEncoding.test_auto_decode_utf_16_le test was failing on Big Endian
systems, such as Fedora's s390x builders. A similar test, but with BE BOM
test_auto_decode_utf_16_be was added in order to reproduce this on a Little
Endian system (which is much easier to come by).

A regression test was added to check that all listed encodings in
utils.encoding.BOMS are valid.

Fixes https://github.com/pypa/pip/issues/6054