Closes #2111

isPinnedVersion -> isVersion
rangify -> getNewValue

Renames currentVersion to currentValue, newVersion to newValue, newVersionMajor to newMajor, and newVersionMinor to newMinor.

Adds support for upgrading equals values, e.g. from `=1.0.0` to `=1.0.1`.

If a current version is missing, we now return both rollback as well as upgrades if upgrades are present.

The goal of this PR is to refactor npm's version lookup/grouping as much as possible to enable the same logic to be used for other package managers.