Fix plugins versions being alpha compared

[itowlson]

Fixes #2946

[vdice]

I'm still confused on how the previous logic incorrectly determined 0.9.0 to be greater than 0.10.0... in any case, I wonder if now might be a good time to add a few unit tests here? (Or do we already have some?)

[itowlson]

@vdice The previous logic used lexical comparison of the version strings. 0=0, .=., but 9>1. (The PluginManifest::version field is a String not a Version, I assume because it is a serialisation type and needs to be forgiving of strings like "canary".)

There's a lot of places I would like to add unit tests for the plugin manager, but it requires some significant refactoring to do that. For example, the function I change here doesn't just validate the installability of the plugin, it also reads from disk and prints to the terminal. So testing it becomes a pain. I can pull out the bit that checks if this is a downgrade and test that function, but at that point we're barely above the level of testing if semver::Version implements Ord correctly. So: I agree: this does need a test; but I am not sure how to write a useful one in the current code structure.

[kate-goldenring]

Thank you for fixing this @itowlson