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.

testdata/go1.22/out.txt

@@ -0,0 +1,12 @@
+10
+9
+8
+7
+6
+5
+4
+3
+2
+1
+go1.22 has lift-off!
+loops behave like Go 1.22