Re-extract DependencyResolver#find_possible

This was removed by @a0f0de7 but I have restored it.
Improve variable names in resolve_for

"possible" is now "matching" and "all_possible" is now "all".
Include other possible names in ActivationRequest

This will improve debugging messaging.
Extract DependencyResolver#activation_request

This moves all ActivationRequest creation to one call site.

Strangely, it requires a change to
test_raises_when_possibles_are_exhausted, but the change seems
acceptable.

@evanphx should review this commit.
Add missing InstalledSpecification#set

This prevented pretty_print from working correctly.