swift-package-manager

[DependencyResolver] Add initial cut at primary resolution loop.

This is the beginnings of a bottom-up constraint solver, but backtracking is
unimplemented.

I plan to document the motivations for taking this resolution strategy in more
detail, but for now the jist is below.

The assumption based on the problem domain is that the most logical notion to
the user is that of resolving the dependencies for an individual package in
isolation, simply because each dependency is presumably developed by someone,
and that is the situation that that developer is in.

Given that assumption, it is useful to frame the dependency resolver in terms of
solving individual packages with some amount of additional constraints imposed
(those additional constraints represent the impact of other dependencies in the
graph on that individual package). I believe this will allow us to give good
diagnostics of the form:

  Dependency A: 1.0.0 ..< 2.0.0 could not be resolved with the following additional constraints:
    B_0: ... 
    ...
    B_N: ...

and then we can provide tools so that the author of `A` can trial the resolution
of their package with those additional constraints active. This is designed to
allow the *author* of the dependency which is causing dependency resolution
issues to be in a good position to resolve the dependency problem (without
needing to necessarily examine the client package, which may not be accessible
to them).

It remains to be seen how efficiently we can implement this strategy, but my
motivation is to build an algorithm which gives good diagnostics first, and
focus on performance second.