properly support multiple archive files

Currently, apkg make-archive and get-archive can in theory download additional files such as signatures but srcpkg/build only accept a single archive file through CLI arguments - it isn't clear how apkg should handle additional files needed with archive.

For example, rpm pkgstyle requires archive to be copied in specific SOURCES dir but apkg doesn't know which additional files should be copied alongside the source archive.

I see following approaches:

1. hacky: assume all files starting with archive filename are signatures and copy them alongside archives.

2. config: specify additional files / signatures in apkg config and look for them alongside archives

3. universal: pass full list of archive files to srcpkg/build --archive, i.e.:

apkg srcpkg -a ~/dl/foo-1.0.tar.gz ~/dl/foo-1.0.tar.gz.asc

This is a prerequisite for #3.

In case 3) is implemented, consider making build --srcpkg behave the same way on source packages.