Add a goal order parameter to the dependency solver

[grayjay]

This PR implements #3489. It isn't ready to be merged yet, because I need to finish updating the tests that are affected by goal order.

I wanted to make it available for review before finishing it, because it probably overlaps with #3502. The second commit exposes the solver's goal qualification types. I moved types like Q, QPN, and PP into a new module, D.Solver.Types.PackagePath, and renamed some of them. /cc @dcoutts

[dcoutts]

Second patch looks fine to me.

[23Skidoo]

/cc @kosmikus

[grayjay]

Thanks! I made a separate PR with the second commit: #3511

[grayjay]

I rebased on top of #3511 and used the new parameter in the three unit tests that seemed most fragile.

/cc @edsko @kosmikus

[kosmikus]

LGTM.

[dcoutts]

@23Skidoo since @grayjay and @kosmikus are happy with it then it's probably good to go.

[23Skidoo]

Merged, thanks!

[grayjay]

Thank you!