Replace `Dependency.merge(Dependency)` with `Dependency.matches(Dependency)`

[kinke]

[PetarKirov]

@kinke @Geod24 What's the status of this PR?

[Geod24]

Looks pretty good to me, was waiting until @kinke takes it out of draft, but the idea is sound.

[kinke]

TODO:

• Decide whether to keep the old .merge() (for potential API users - reggae doesn't need it AFAICT)
• Decide if treating all paths as equivalent is good enough for now (we get a bunch of new warnings at Symmetry with dub master regarding ../pkg1 vs. ../../pkg1 etc., which would be eliminated by doing so)
• Decide if the changed priorities (path-based dominating repos) are a benign 'breaking' change
• Look into the unittest failure wrt. 1.0.0 not matching >=1.0.0 <2.0.0, though not really related to this PR
• Decide how to handle 2 actual VersionRanges, i.e., whether a non-empty intersection is good enough, or whether the lhs should be guaranteed to be a single version (the selected version). This requires looking into how DependencyResolver.matches() is used (WTF are configs and config).