Add support for extension .mo in [civicrm.l10n]

#30 (closed) added support for translation "mo" files in a custom resource directory (such as files/civicrm/l10n). This makes it easier to update files regularly, such as with the l10nupdate extension, which must be able to write to disk.

However currently extensions must still have their "mo" files in their own directory. This causes challenges with the distribution of mo files for core extensions, but it is also annoying for non-core extensions that are available for automatic distribution (i.e. on Transifex).

Proposal: support both, as a transitional measure.

Eventually, I think we should deprecate the old behaviour, and always have a process such as l10nupdate which will fetch translations. That way we can also deprecate civicrm-l10n-xx.tar.gz.