add tests for get from tar subdir

Test that we can get a subdirectory from a tarball (or any other
"packed" source that we support).

The 'tar-subdir-to-parent' test highlights a regression where the
subdirectory module references a module in its parent directory. This
breaks the intended use ofr the subdirectory and the implementation in
go-getter. We need to fix this in terraform, and possible plan warnings
and deprecations for this type of source.

config/module/test-fixtures/basic-tar-subdir/main.tf

@@ -0,0 +1,3 @@
+module "foo" {
+    source = "./foo.tgz//sub"
+}

config/module/test-fixtures/tar-subdir-to-parent/main.tf

@@ -0,0 +1,4 @@
+module "foo" {
+	// the module in sub references sibling module baz via "../baz"
+    source = "./foo.tgz//sub"
+}

config/module/tree_test.go

@@ -209,40 +209,50 @@ func TestTreeLoad_parentRef(t *testing.T) {
 }
 
 func TestTreeLoad_subdir(t *testing.T) {
-	storage := testStorage(t)
-	tree := NewTree("", testConfig(t, "basic-subdir"))
-
-	if tree.Loaded() {
-		t.Fatal("should not be loaded")
+	fixtures := []string{
+		"basic-subdir",
+		"basic-tar-subdir",
+		"tar-sbudir-to-parent",
 	}
 
-	// This should error because we haven't gotten things yet
-	if err := tree.Load(storage, GetModeNone); err == nil {
-		t.Fatal("should error")
-	}
+	for _, tc := range fixtures {
+		t.Run(tc, func(t *testing.T) {
+			storage := testStorage(t)
+			tree := NewTree("", testConfig(t, tc))
 
-	if tree.Loaded() {
-		t.Fatal("should not be loaded")
-	}
+			if tree.Loaded() {
+				t.Fatal("should not be loaded")
+			}
 
-	// This should get things
-	if err := tree.Load(storage, GetModeGet); err != nil {
-		t.Fatalf("err: %s", err)
-	}
+			// This should error because we haven't gotten things yet
+			if err := tree.Load(storage, GetModeNone); err == nil {
+				t.Fatal("should error")
+			}
 
-	if !tree.Loaded() {
-		t.Fatal("should be loaded")
-	}
+			if tree.Loaded() {
+				t.Fatal("should not be loaded")
+			}
 
-	// This should no longer error
-	if err := tree.Load(storage, GetModeNone); err != nil {
-		t.Fatalf("err: %s", err)
-	}
+			// This should get things
+			if err := tree.Load(storage, GetModeGet); err != nil {
+				t.Fatalf("err: %s", err)
+			}
 
-	actual := strings.TrimSpace(tree.String())
-	expected := strings.TrimSpace(treeLoadSubdirStr)
-	if actual != expected {
-		t.Fatalf("bad: \n\n%s", actual)
+			if !tree.Loaded() {
+				t.Fatal("should be loaded")
+			}
+
+			// This should no longer error
+			if err := tree.Load(storage, GetModeNone); err != nil {
+				t.Fatalf("err: %s", err)
+			}
+
+			actual := strings.TrimSpace(tree.String())
+			expected := strings.TrimSpace(treeLoadSubdirStr)
+			if actual != expected {
+				t.Fatalf("bad: \n\n%s", actual)
+			}
+		})
 	}
 }