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/dagger.cue

@@ -5,6 +5,16 @@ import (
 	"alpha.dagger.io/dagger/op"
 )
 
+// A reference to a filesystem tree.
+// For example:
+//  - The root filesystem of a container
+//  - A source code repository
+//  - A directory containing binary artifacts
+// Rule of thumb: if it fits in a tar archive, it fits in a #FS.
+#FS: {
+	_fs: id: string
+}
+
 // An artifact such as source code checkout, container image, binary archive...
 // May be passed as user input, or computed by a buildkit pipeline
 #Artifact: {
@@ -21,7 +31,12 @@ import (
 	id: string
 }
 
-// Secret value
+// A reference to an external secret, for example:
+//  - A password
+//  - A SSH private key
+//  - An API token
+// Secrets are never merged in the Cue tree. They can only be used
+// by a special filesystem mount designed to minimize leak risk.
 #Secret: {
 	@dagger(secret)
 

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)

stdlib/dagger/plan.cue

@@ -20,7 +20,7 @@ package dagger
 		path: string
 		include?: [...string]
 		exclude?: [...string]
-		fs: #Artifact
+		fs: #FS
 	}
 
 	// Securely load external secrets