loader: make sure Go version is plumbed through

This fixes the new loop variable behavior in Go 1.22.

Specifically:
  * The compiler (actually, the x/tools/go/ssa package) now correctly
    picks up the Go version.
  * If a module doesn't specify the Go version, the current Go version
    (from the `go` tool and standard library) is used. This fixes
    `go run`.
  * The tests in testdata/ that use a separate directory are now
    actually run in that directory. This makes it possible to use a
    go.mod file there.
  * There is a test to make sure old Go modules still work with the old
    Go behavior, even on a newer Go version.

loader/loader_go122.go

@@ -0,0 +1,17 @@
+//go:build go1.22
+
+// types.Info.FileVersions was added in Go 1.22, so we can only initialize it
+// when built with Go 1.22.
+
+package loader
+
+import (
+	"go/ast"
+	"go/types"
+)
+
+func init() {
+	initFileVersions = func(info *types.Info) {
+		info.FileVersions = make(map[*ast.File]string)
+	}
+}