module: basic module.Version type and support

Copied from cmd/go/internal/module and added Version.String method.

For golang/go#31761.

Change-Id: I2fbe224843378e68f0e5890dbc36766936a7e3ed
Reviewed-on: https://go-review.googlesource.com/c/mod/+/176461
Reviewed-by: Hyang-Ah Hana Kim <[email protected]>

Author: rsc