cmd/go: factor out internal/semver, internal/modfile etc #28101
Description
The parsing functionality found in the modfile package is useful to external tools
(for example for tools that wish to edit go.mod files in ways not supported by go mod edit, such as gohack).
The semver functionality found in semver is not quite the same as other semver packages, and is likewise useful for external packages.
There are already at least two repositories containing copies of these packages 1 2. It would be nice if there was a commonly recognized place for them.