cleanup: solver/fs

- Solver: Encapsulates all access to Buildkit. Can solve plain LLB, invoke external frontends (for DockerBuild) and export (for ContainerPush)
- FS (now BuildkitFS) implements the standard Go 1.16 io/fs.FS interface and provides a read-only filesystem on top of a buildkit result. It can be used with built-ins such as fs.WalkDir (no need to have our own Walk functions anymore)
- Moved CueBuild into compiler.Build since it no longer depends on Buildkit. Instead it relies on the io/fs.FS interface, which is used both for the base config and the stdlib (go:embed also uses io/fs.FS). Overlaying base and the stdlib is now done by the same code.

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

stdlib/stdlib.go

@@ -2,48 +2,14 @@ package stdlib
 
 import (
 	"embed"
-	"fmt"
-	"io/fs"
 	"path"
-	"path/filepath"
-
-	cueload "cuelang.org/go/cue/load"
 )
 
-// FS contains the filesystem of the stdlib.
-//go:embed **/*.cue **/*/*.cue
-var FS embed.FS
+var (
+	// FS contains the filesystem of the stdlib.
+	//go:embed **/*.cue **/*/*.cue
+	FS embed.FS
 
-const (
-	stdlibPackageName = "dagger.io"
+	PackageName = "dagger.io"
+	Path        = path.Join("cue.mod", "pkg", PackageName)
 )
-
-func Overlay(prefixPath string) (map[string]cueload.Source, error) {
-	overlay := map[string]cueload.Source{}
-
-	err := fs.WalkDir(FS, ".", func(p string, entry fs.DirEntry, err error) error {
-		if err != nil {
-			return err
-		}
-
-		if !entry.Type().IsRegular() {
-			return nil
-		}
-
-		if filepath.Ext(entry.Name()) != ".cue" {
-			return nil
-		}
-
-		contents, err := FS.ReadFile(p)
-		if err != nil {
-			return fmt.Errorf("%s: %w", p, err)
-		}
-
-		overlayPath := path.Join(prefixPath, "cue.mod", "pkg", stdlibPackageName, p)
-
-		overlay[overlayPath] = cueload.FromBytes(contents)
-		return nil
-	})
-
-	return overlay, err
-}