Define relationship between preferred versions and ToChange/ChangeAll params

[fabulous-gopher]

From @sdboyer on July 12, 2016 2:7

Right now, preferred versions get added to the queue without regard for the ToChange and ChangeAll params. I'm not completely sure what the best behavior here is, but I think it should probably look like this:

• If ChangeAll is not set and ToChange does not have a given project in it, then add prefv to version queue as normal
• If ChangeAll IS set and ToChange does not have a given project in it, AND the root neither imports nor constrains the project (aka, it is entirely a transitive dep), then add prefv to version queue. This allows general updates to maintain stability in projects the user isn't directly paying attention to.
• Regardless of ChangeAll or the root import/constraints, if ToChange is set with the project name, then don't add prefv to the version queue. (Explicit user input always wins)

Copied from original issue: sdboyer/gps#64