dagger compute auto-fetches standard library from external repo

- Standard library is hosted at github.com/blocklayerhq/dagger-stdlib
- When dagger repo is made public, stdlib can be hosted from same repo
- Vendored cue.mod is merged with stdlib at file granularity
- When developing dagger, set DAGGER_DEV_STDLIB to a local stdlib
  directory

Signed-off-by: Solomon Hykes <sh.github.6811@hykes.org>

dagger/pipeline.go

@@ -231,7 +231,13 @@ func (p *Pipeline) Local(ctx context.Context, op *cc.Value) error {
 	if err := op.Get("include").Decode(&include); err != nil {
 		return err
 	}
-	p.fs = p.fs.Set(llb.Local(dir, llb.FollowPaths(include)))
+	p.fs = p.fs.Change(func(st llb.State) llb.State {
+		return st.File(llb.Copy(
+			llb.Local(dir, llb.FollowPaths(include)),
+			"/",
+			"/",
+		))
+	})
 	return nil
 }