dagger.#FS support

- Implement dagger.#FS support
- Migrate `context.imports` to dagger.#FS
- Backward compat: dagger.#FS can be passed in lieu of a
  dagger.#Artifact
- For instance, an import (`dagger.#FS`) can be passed to the current
  `yarn.#Package` implementation

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

stdlib/dagger/op/op.cue

@@ -20,8 +20,10 @@ package op
 }
 
 #Local: {
-	do: "local"
-	id: string
+	do:  "local"
+	dir: string
+	include: [...string]
+	exclude: [...string]
 }
 
 // FIXME: bring back load (more efficient than copy)