add option to build dependencies only

[kkendzia]

I have a use case where it's only necessary to build dependencies without the package itself. I tried to added the option, but I am not very pleased with the _checkDepsOnly function. Maybe you have an idea.

[codecov-io]

Codecov Report

Merging #221 into master will increase coverage by 0.15%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master     #221      +/-   ##
==========================================
+ Coverage   74.07%   74.23%   +0.15%     
==========================================
  Files          26       26              
  Lines        9297     9315      +18     
==========================================
+ Hits         6887     6915      +28     
+ Misses       2410     2400      -10

Impacted Files	Coverage Δ
pym/bob/input.py	87.6% <ø> (+0.1%)	⬆️
pym/bob/cmds/build.py	85.66% <96.29%> (+0.21%)	⬆️
pym/bob/audit.py	63.86% <0%> (+2.91%)	⬆️

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[jkloetzke]

I'm wondering if this shouldn't be realized with the query language. In principle bob build '<path-to-package>/*' should already be almost what you want. The only thing that might not be built are some tools that are used by the package but are not used by any of it's dependencies. This reminds me of #187 which is basically the same problem.

Maybe something like bob build '<path-to-package>/deps@*'. What do you think?

[kkendzia]

Would be fine for me! Yeah I also tried /* and encountered the same issue with tools.

[kkendzia]

Mh do you think /child@* should work too?

Edit: Forget it, just tried it

[jkloetzke]

I did have a look and solving the problem with the query language is not so easy as it seems. Nonetheless I think this is the correct approach given that #187 needs internally the same information. I would rather not solve this only for the local build with this PR.

Still, many thanks for submitting the PR in the first place.