Support semver prerelease versions

The semver library's compare functions already did the right thing,
but our `PackageVersion.versionMagnitude` did not. It now almost
correctly works for prerelease versions with a few caveats described
in a [*] comment.

While at it, add many tests which caught a separate bug due to
adding a number to a string for versions such as "1.2.3_50".
(That version would have been chosen over "5.0.0"!)