Add example of how to depend one pkg_deb .changes file

[aiuto]

• Improve the example about finding output file paths to show how to
  get the implicit outputs like the changes file.

Fixes #477

cc/ @phlax

[phlax]

thanks for this @aiuto i have tested it and, as suggested, using filegroup to pull out the debs and changes files, it works a treat

[nacl]

I will find time to review this today or Friday.

[aiuto]

@phlax Was the example clear enough?

[phlax]

Was the example clear enough?

yep. also, happy to relate use...

first, i tried to just use the pkg_deb rule i have as a src for a pkg_tar rule

this gave me the files i wanted, but it also added unwanted rulename.deb.deb files

so i then added a filegroup for each output_group of deb and changes which gave me the specific files i wanted when added to my pkg_tar

the only mistake i initially made was in thinking that it required a pkg_filegroup rather than a filegroup but not sure if that actually needs to be disambiguated, just my confusion

hope this helps

[nacl]

The documentation updates are very useful, but I don't think we've ever quite come to a consensus on what should be in DefaultInfo. See the large comment for a starter discussion. on that.

I think what you have specified here is the right thing to do, but I am not completely sold on it. The code looks good modulo the below small suggestions in the test code.

[aiuto]

@nacl If you could do a quick review again, I would like to get this in for the example, as it appears relevant to bazelbuild/bazel#7977

[nacl]

LGTM. Please update/reword the commit message to indicate what actually is happening here -- just a documentation/examples update.