go/packages: pass go list-reported Go language version to type checker

Type checking of a package depends on the Go language version in
effect for that package. We have been not setting it and assuming
"latest" is good enough, but that is likely to become untrue in the
future, and it violates Go 1.21's emphasis on forward compatibility,
namely tools recognizing when they shouldn't be processing newer code.

Pass the Go version along from go/packages to go/types, to allow
go/types to apply the version when type-checking.

This is tested by CL 507880.

For golang/go#61174.

Change-Id: I49353dede9c7c095c2cd0c4a6959f9f6e6a06ec5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/507879
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

go/packages/packages.go

@@ -1043,6 +1043,9 @@ func (ld *loader) loadPackage(lpkg *loaderPackage) {
 		Error: appendError,
 		Sizes: ld.sizes,
 	}
+	if lpkg.Module != nil && lpkg.Module.GoVersion != "" {
+		typesinternal.SetGoVersion(tc, "go"+lpkg.Module.GoVersion)
+	}
 	if (ld.Mode & typecheckCgo) != 0 {
 		if !typesinternal.SetUsesCgo(tc) {
 			appendError(Error{