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>

dagger/compiler/build.go

@@ -0,0 +1,63 @@
+package compiler
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"path"
+	"path/filepath"
+
+	cueerrors "cuelang.org/go/cue/errors"
+	cueload "cuelang.org/go/cue/load"
+)
+
+// Build a cue configuration tree from the files in fs.
+func Build(sources map[string]fs.FS, args ...string) (*Value, error) {
+	buildConfig := &cueload.Config{
+		// The CUE overlay needs to be prefixed by a non-conflicting path with the
+		// local filesystem, otherwise Cue will merge the Overlay with whatever Cue
+		// files it finds locally.
+		Dir:     "/config",
+		Overlay: map[string]cueload.Source{},
+	}
+
+	// Map the source files into the overlay
+	for mnt, f := range sources {
+		f := f
+		mnt := mnt
+		err := fs.WalkDir(f, ".", 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(f, p)
+			if err != nil {
+				return fmt.Errorf("%s: %w", p, err)
+			}
+
+			overlayPath := path.Join(buildConfig.Dir, mnt, p)
+			buildConfig.Overlay[overlayPath] = cueload.FromBytes(contents)
+			return nil
+		})
+		if err != nil {
+			return nil, err
+		}
+	}
+	instances := cueload.Instances(args, buildConfig)
+	if len(instances) != 1 {
+		return nil, errors.New("only one package is supported at a time")
+	}
+	inst, err := Cue().Build(instances[0])
+	if err != nil {
+		return nil, errors.New(cueerrors.Details(err, &cueerrors.Config{}))
+	}
+	return Wrap(inst.Value(), inst), nil
+}