Instead, perform isVersion filter in lookup

Adds pypi datasource, compatible with existing npm and github ones. Performs no validation/filtering of versions.

Closes #1764 

Corrected logic mistake and renamed unpublishable (internal) to canBeUnpublished to make it easier to follow logic.

Closes #2080

Closes #2062

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`.