Add back support for full VersionNumbers to the resolver

[KristofferC]

We are getting quite a few requests for various functionalities that cannot really be implemented due to the resolver right now only supporting major.minor.patch versions. These are things like:

• Pre-release support
• Build meta-data

Ironically, the resolver used to have support for the full version number spec but this was removed due to the belief that it was not useful in Pkg3 (f5eff12).

@carlobaldassi, just checking, what level of effort do you think it is to put back the prerelease/build stuff to the resolver? If that support was added back I think I could do the rest of the surrounding scaffolding to add e.g. pre-release support.

[carlobaldassi]

I just had a look at the diff in that commit you linked, in which the functionality was removed. In principle, it should be sufficient to simply revert that. (It may slow down the resolver a bit though, and eat up some extra memory, I'm not sure to what extent.) I believe there were also some tests, removed in some other commit.

The code used to support also versions of the form 1.2.3- and 1.2.3+ which are not valid semver but were used to specify ranges (and indeed are still accepted as VersionNumber objects in Julia). Also, since we need to rank all versions somehow, the code ignores the semver prescription that build meta-data should not be used in the ranking; instead, it applies the same logic as for prerelease specs (1.2.3+beta > 1.2.3+alpha > 1.2.3).