repositories: Refactor how we are reading metadata

repositories: Refactor how we are reading metadata

This is a large rewrite of how we are discovering and reading any
repository metadata.

It first of all makes the code a little bit more straight forward by
breaking steps into their own function.

Those functions will now do "the right thing" depending whether we are
dealing with a local or remote repository and will try to read
repository metdata for local repositories, too.

If that fails, we will of course fall back and scan.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>