Our current filtering mechanism did not allow for plain
emails as uids. Which is what can be done and some people do it.

To safely import keys where there is a plain email in the uid,
we can make use of the new mbox import filter, that is available
since 2.1.14.

With this we can change our filtering (through import & export)
approach:

1. we detect if we have a single key through simple
   `gpg < keymaterial` inspection.
2. if we have a single key, we record the fingerprint
3. We import using the mbox filter with a direct match of the email
   address
4. We export by matching with the fingerprint

This will give us the exact matching key (through fpr) if there is
a direct match on the email, which is all delegated to gpg itself.