pkg2/resolve: add prerelase/build in VersionWeigth

this will allow to remove filtering of prerelease/build versions,
and "normalization" of packages requirements which was introduced
in 0fa84ec69f060cc492772b1a1707eff3745675bf
pkg2: reinstate prerelease/build versions

Prerelease/build versions can again be used in "require" files.

This essentially reverts commits
0fa84ec69f060cc492772b1a1707eff3745675bf
923e385d029b22df67739f7f1de6df18721d2f97
pkg2: more tests in resolve2

For the new weighting scheme. Also, sanitize previous
tetst a little.
Merge pull request #3898 from carlobaldassi/pkg2moreweights

Pkg2: reinstate prerelease/build versions