RFE: accept URLs for input files

In order to provide users with maximum power, apkg commands should accept URLs as its input files (--archive, --srcpkg, ...) and automatically download files in question into appropriate location (pkg/remote or pkg/downloads).

This will allow for example single command package build directly from an arbitrary archive on the internet:

apkg build -b --upstream --archive https://secure.nic.cz/files/knot-resolver/knot-resolver-5.5.0.tar.xz

As multiple files with the same name can be downloaded from different URLs, the path needs to get a little ugly with a URL hash:

pkg/remote/de23ad/knot-resolver-5.5.0.tar.cz

This raises questions regarding caching:

• Should apkg cache downloaded files or (re-)download on each invocation?
• Should it honor --no-cache which is disabled outside of VCS?

This should preferably be implemented after or alongside #73 (closed) cache refactor.