lei: forget-external support with canonicalization

lei: forget-external support with canonicalization

For proper matching, we'll do a better job canonicalizing
URLs and path names for matching.  Of course, users may edit
the file outside of lei, so ensure we try both the canonicalized
and as-is form provided by the user.

I also don't think we'll need to store externals info in
MiscIdx; just the config file is fine.