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>

plancontext/fs.go

@@ -0,0 +1,32 @@
+package plancontext
+
+import (
+	"sync"
+
+	bkgw "github.com/moby/buildkit/frontend/gateway/client"
+)
+
+type FS struct {
+	Result bkgw.Reference
+}
+
+type fsContext struct {
+	l     sync.RWMutex
+	store map[ContextKey]*FS
+}
+
+func (c *fsContext) Register(fs *FS) ContextKey {
+	c.l.Lock()
+	defer c.l.Unlock()
+
+	id := hashID(fs)
+	c.store[id] = fs
+	return id
+}
+
+func (c *fsContext) Get(id ContextKey) *FS {
+	c.l.RLock()
+	defer c.l.RUnlock()
+
+	return c.store[id]
+}