The packer plugins remove command allows users to delete plugins
installed locally.
Previous versions of the command only allowed for the plugins to be
removed using the source for a plugin, and the versions to remove,
optionally.
This commit adds the capability for the plugins to be removed using
their local path, in addition to the regular source+version method, that
way we are able to pipe the results of `packer plugins installed' into
the plugins remove command for quick plugin removal.
930b6c3e2b