[analyzer] Switch language version usages of `Version` to `LanguageVersion`

[parlough]

The analyzer and its siblings should consider switching from Version from package:pub_semver to LanguageVersion from package:package_config when it needs to represent a language version, including for experimental features. The type is lighter weight and more semantic.

Where needed, an extension to convert between the two types can be added and used.

[FMorschel]

FYI @pq

[pq]

Great idea and thanks for the heads-up @FMorschel! Since I'm kind of in here already, I'll take a look at this.

Thanks!

[pq]

@srawlins: did uses of pub_semver Version make their way into the plugin API?

[srawlins]

Not as far as I know.