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>

plan/task/util.go

@@ -0,0 +1,14 @@
+package task
+
+import (
+	"fmt"
+
+	"github.com/moby/buildkit/client/llb"
+	"go.dagger.io/dagger/compiler"
+)
+
+func withCustomName(v *compiler.Value, format string, a ...interface{}) llb.ConstraintsOpt {
+	prefix := fmt.Sprintf("@%s@", v.Path().String())
+	name := fmt.Sprintf(format, a...)
+	return llb.WithCustomName(prefix + " " + name)
+}