Add auth helper and reuse it in git downloader
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Load scripts without preloading a composer instance to avoid side-effects, fixes #3155
Fixed composer.lock hash
Merge pull request #3170 from markuspoerschke/feature/fix-composer-lock

Fixed composer.lock hash
added test unlinking directory

#3157
rmdir needs to be used on windows to remove symbolic links to directories
added test removing directory with trailing slash that is symlinked

#3144
#3157
unlinking symlinked directories and trailing slashes

Filesystem::removeDirectory() didn't detect all symlinked directories
properly due to not resolving pathnames with trailing slashes first.

this commit fixes that issue by resolving pathnames with trailing slashes
by removing those in case they are representing a symlinked directory.

#3144

Reference:

 A.4.12 Pathname Resolution - The Open Group Base Specifications Issue 7
 IEEE Std 1003.1, 2013 Edition Section
 <http://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap04.html>