warn if an expected file can't be acquired

If we couldn't find an entry for a Sources file we would generate an
error while for a Packages file we would silently skip it due to
assuming it is missing because it is empty. We can do better by checking
if the repository declares that it supports a component we want to get
the file from and if not say so and hint at the user making a typo.

An example were this helps is mozilla.debian.net which dropped the
firefox-aurora component (as upstream did) meaning no upgrades until the
user notices manually that the repository doesn't provide packages
anymore. With this commit warnings are raised hopefully causing the user
to investigate what is wrong (sooner).

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