Make on disk git repository names case-insensitive

[schwern]

Github repository names are case insensitive. Perl namespaces are case sensitive. It's possible to have multiple on disk git repositories which are really the same Github repository causing wasted space and maybe repository corruption.

• Make the on-disk git repository directory case insensitive, probably by lower casing the repository name.
• Leave the Github repository name alone.

Leave in a warning about finding multiple distributions with the same name but different case. The Github casing is canonical.

[schwern]

No need for this, the check in import_releases added by #80 makes this moot.