Fix virtual package resolution

[dillon-giacoppo]

Currently, a simple install of bash produces the following error:

WARNING: Following dependencies could not be resolved for bash: awk

In identifying the root cause of this issue, found a few behaviors for the Provides key are not correct (as described by the spec here).

Fixed the following in Provides:

• Handle multiple packages, i.e. Provides: bar, baz.
• Handle unversioned packages, i.e. Provides: bar and Provides: bar (= 1.0) are both valid.
• Ensure alternatives (referred in code as dependency groups) are evaluated left-to-right and added to unmet_deps if none resolve.
• Handle multiple candidates for virtual packages if can fallback to a single Priority: required package.
• Resolve multiple virtual package candidates from the same package to the highest available version.

The usage of Priority: required is necessary to account for Distroless base images not containing the "true" minimal package set as described here: https://wiki.debian.org/Proposals/EssentialOnDiet. This creates a problem for package dependencies like awk which expect to find mawk by default. A potential future improvement could resolve ambiguous virtual packages by picking an explicit package in the manifest.

Fixes to the Provides handling leads to significantly more virtual packages being identified and resolved correctly.

[thesayyn]

This is awesome, can we drop some links here from the upstream debian documentation/tests for future references?

[thesayyn]

There seems to be lots of improvements here. It might take me couple days to understand this.

[thesayyn]

There are some buildifier issues, can you look at them?