Store dependency distances on DependencyDescriptors

[cottsay]

Store dependency distances on DependencyDescriptors

The distance measures the smallest number of steps that each recursive dependency is from the package.

Depends on #173
Depends on #174

[codecov-io]

Codecov Report

Merging #172 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
+ Coverage   77.94%   77.98%   +0.03%     
==========================================
  Files          53       53              
  Lines        3034     3039       +5     
  Branches      507      507              
==========================================
+ Hits         2365     2370       +5     
  Misses        641      641              
  Partials       28       28

Impacted Files	Coverage Δ
colcon_core/package_descriptor.py	100% <100%> (ø)	⬆️

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[cottsay]

Hold off on this - I need to work out an issue.

[cottsay]

Okay, I worked out the kinks. The other two PRs should be reviewed before this one.

[dirk-thomas]

With the patch looking almost ready I was wondering about if it is a good idea to calculate the depth information in colcon-core if only colcon/colcon-package-selection#22 wants to use it. I will need to think about what it would take to push this out into the extension actually needing the information...

[cottsay]

I'm open to other approaches. This solution should only be adding to the size of the descriptors - it shouldn't be increasing the computational complexity of the topographical ordering algorithm. I changed the algorithm to work in "layers", but it should still be doing the same amount of work.

[dirk-thomas]

I looked into moving the depth logic into colcon-package-selection (close to where it is actually be used for the new argument). The problem with this is that the extension doesn't know what kind of dependency categories where used when calculating the recursive dependencies. So it can't recreate them there (without that knowledge). So while I am not a big fan of calculating the depth here (since it is only used in one extension in another repo) I don't see a reasonable way to do it differently atm.