engine: resources: Detect simple self-referential frags

It would be a likely mistake to create a self-referential frag, and mgmt
would spin forever updating the file... We probably don't want this, so
let's just catch this case in Validate.

Of course you could get around this with multiple files, and a fancier
search could statically check the graph, but the goal isn't to prevent
any bad code, since that's not likely to be possible.

lang/interpret_test/TestAstFunc3/frag1.txtar

@@ -0,0 +1,16 @@
+-- main.mcl --
+file "/tmp/frags/" {
+	state => "exists",
+}
+file "/tmp/frags/f1" {
+	state => "exists",
+	content => "f1 contents\n",
+}
+file "/tmp/frags/all" {
+	state => "exists",
+	fragments => [
+		"/tmp/frags/",
+	],
+}
+-- OUTPUT --
+# err: errValidate: file[/tmp/frags/all] did not Validate: inside a frag (`/tmp/frags/`)