Don't throw away alternate sources for transitive deps during import

[carolynvs]

During init, when we are importing external config, dep throws away all constraints for transitive dependencies. This is a problem when the config specified an alternate source. In this case, we should create an override with just the source set, e.g.

[[override]]
  name = "github.com/pkg/X"
  source = "https://example.com/pkg/X.git"

This is part of epic #845.

[sudo-suhas]

Hey @carolynvs, I'd like to work on this. Could you help me figure out how to setup and test a scenario where this would be relevant? As for the check for alternate source, would it be correct to compare the project root and it's Source specified in the ProjectProperties?

[carolynvs]

@sudo-suhas I've been in some design sessions recently and the design for this issue is currently undecided. I've flagged this as on-hold and hopefully I'll have a decision in a couple weeks. In the meantime, would you be interested in taking #1315 instead?

[sudo-suhas]

Sure @carolynvs, I'll do that. I'll also comment there to let people know somebody is working on it..