collectConstraints() should collect applicable constraints only

[darkowlzz]

#962 added collectConstraints() for collecting constraints of all the projects that are direct dependencies, but it collects constraints that are not applicable too. It should only collect applicable project constraints.

Refer: https://github.com/golang/dep/pull/962/files#r153268638

[JackyChiu]

I can hop on this @darkowlzz 😄

[darkowlzz]

@JackyChiu Great! Go for it.
But before you raise a PR, be aware of the in-flight PR, which is almost ready, that would change the code related to this fix. Since the actual algorithm is still the same, you can figure out the fix and rebase later if required.

[JackyChiu]

Hey @darkowlzz could you clarify what an "applicable" constraint is?

[darkowlzz]

@JackyChiu yes, so applicable constraints are the working constraints, which are actually imported in the project by their import paths. If you check in getApplicableConstraints(), it first collects all the constraints in manifest (constraints and overrides), and then it goes through all the import paths in the project, and marks the constraints as working constraints if they are imported in the project. The constraints that don't have any imports in the project are not applicable constraints and we don't include them.

Hope it made sense 😁 I can explain again with more examples if required.

[JackyChiu]

@darkowlzz Thanks for explanation, will try tonight since the other PR got merged