Fix RepoConfigLoader so that it's possible to use PORTDIR_OVERLAY

Fix RepoConfigLoader so that it's possible to use PORTDIR_OVERLAY
to override the repo that PORTDIR refers to.

This fixes a regression which prevented a common repoman use case
from working in which the user has PORTDIR refering to a rsync
tree and has a separate cvs tree with the same repo_name as
PORTDIR. When the user tries to use repoman with the cvs tree,
the cvs repo is appended to PORTDIR_OVERLAY and we want it to
override the conflicting PORTDIR setting.