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/oldgo/go.mod

@@ -0,0 +1,5 @@
+module github.com/tinygo-org/tinygo/testdata/oldgo
+
+// Go version doesn't matter much, as long as it's old.
+
+go 1.15

testdata/oldgo/main.go

@@ -0,0 +1,26 @@
+package main
+
+// This package verifies that the Go language version is correctly picked up
+// from the go.mod file.
+
+func main() {
+	testLoopVar()
+}
+
+func testLoopVar() {
+	var f func() int
+	for i := 0; i < 1; i++ {
+		if i == 0 {
+			f = func() int { return i }
+		}
+	}
+	// Variable n is 1 in Go 1.21, or 0 in Go 1.22.
+	n := f()
+	if n == 0 {
+		println("loops behave like Go 1.22")
+	} else if n == 1 {
+		println("loops behave like Go 1.21")
+	} else {
+		println("unknown loop behavior")
+	}
+}

testdata/oldgo/out.txt

@@ -0,0 +1 @@
+loops behave like Go 1.21