Calculate package directory correctly in pip_install

Strip the [<extras>] string from a <package_dir>[<extras>] argument
when looking for the package directory.  Explain what the heck is
going on.

Change-Id: I79beb5c3e9e7c35c91cdd0d5a1d91532bebc4b6d
Closes-Bug: #1721638
1 file changed