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>

docs/reference/dagger/README.md

@@ -20,6 +20,18 @@ _No input._
 
 _No output._
 
+## dagger.#FS
+
+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.
+
+### dagger.#FS Inputs
+
+_No input._
+
+### dagger.#FS Outputs
+
+_No output._
+
 ## dagger.#Plan
 
 A deployment plan executed by `dagger up`
@@ -34,7 +46,7 @@ _No output._
 
 ## dagger.#Secret
 
-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.
 
 ### dagger.#Secret Inputs