go/types: record Config.GoVersion for reporting in Package.GoVersion method

go/types: record Config.GoVersion for reporting in Package.GoVersion method

Clients of go/types, such as analyzers, may need to know which
specific Go version a package is written for. Record that information
in the Package and expose it using the new GoVersion method.

Update parseGoVersion to handle the new Go versions that may
be passed around starting in Go 1.21.0: versions like "go1.21.0"
and "go1.21rc2". This is not strictly necessary today, but it adds some
valuable future-proofing.

While we are here, change NewChecker from panicking on invalid
version to saving an error for returning later from Files.
Go versions are now likely to be coming from a variety of sources,
not just hard-coded in calls to NewChecker, making a panic
inappropriate.

For #61174.
Fixes #61175.

Change-Id: Ibe41fe207c1b6e71064b1fe448ac55776089c541
Reviewed-on: https://go-review.googlesource.com/c/go/+/507975
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>