Fix support for detached upstream signatures

We should only allow upstream tarballs with detached signatures, but not
native tarballs, «<source>_<version>.orig.asc» or «<source>_<version>.asc»
filenames. We also need to allow this on all relevant regexes. This commit
fixes and refactors the regexes to implement the above, and to make it
easier to modify a single place in the future.

Fixes commit 4911282cbf231b116872a6f16eb838f7862ebc36.

Closes: #759401