[remove nixpkgs] Enable installing and removing packages (#1255)

## Summary

builds on top of #1236

Goal: 
We should be able to install the package without downloading nixpkgs.

Changes made to:
- `nixprofile.ProfileInstall`: use `/nix/store/<hash>` as `installable`
- `nixprofile.ProfileListIndex`: return index when `item.nixStorePath ==
pkg.PathInStore`
- `devpkgs.Package.ValidateExists`: `true` if `IsInBinaryStore`
- `devpkgs.Package.EnsureNixpkgsPrefetched`: skip if `IsInBinaryStore`.


## How was it tested?

`devbox add [email protected]` and `devbox rm [email protected]`
- [x] nixpkgs not downloaded

Author: savil

internal/devpkg/package.go

@@ -15,6 +15,7 @@ import (
 
 	"github.com/pkg/errors"
 	"github.com/samber/lo"
+	"go.jetpack.io/devbox/internal/boxcli/featureflag"
 	"go.jetpack.io/devbox/internal/boxcli/usererr"
 	"go.jetpack.io/devbox/internal/cuecfg"
 	"go.jetpack.io/devbox/internal/lock"
@@ -324,6 +325,15 @@ func (p *Package) ValidateExists() (bool, error) {
 	if p.isVersioned() && p.version() == "" {
 		return false, usererr.New("No version specified for %q.", p.Path)
 	}
+
+	isInStore, err := p.IsInBinaryStore()
+	if err != nil {
+		return false, err
+	}
+	if isInStore {
+		return true, nil
+	}
+
 	info, err := p.NormalizedPackageAttributePath()
 	return info != "", err
 }
@@ -378,6 +388,17 @@ func (p *Package) LegacyToVersioned() string {
 }
 
 func (p *Package) EnsureNixpkgsPrefetched(w io.Writer) error {
+
+	isInStore, err := p.IsInBinaryStore()
+	if err != nil {
+		return err
+	}
+	if isInStore {
+		// We can skip prefetching nixpkgs, if this package is in the binary
+		// cache store.
+		return nil
+	}
+
 	hash := p.HashFromNixPkgsURL()
 	if hash == "" {
 		return nil
@@ -408,6 +429,10 @@ func (p *Package) HashFromNixPkgsURL() string {
 const BinaryCacheStore = "https://cache.nixos.org"
 
 func (p *Package) IsInBinaryStore() (bool, error) {
+	if !featureflag.RemoveNixpkgs.Enabled() {
+		return false, nil
+	}
+
 	if !p.isVersioned() {
 		return false, nil
 	}

internal/impl/packages.go

@@ -243,7 +243,7 @@ func (d *Devbox) addPackagesToProfile(ctx context.Context, mode installMode) err
 
 	// If packages are in profile but nixpkgs has been purged, the experience
 	// will be poor when we try to run print-dev-env. So we ensure nixpkgs is
-	// prefetched for all our packages.
+	// prefetched for all relevant packages (those not in binary store).
 	for _, input := range d.PackagesAsInputs() {
 		if err := input.EnsureNixpkgsPrefetched(d.writer); err != nil {
 			return err

internal/nix/nixprofile/profile.go

@@ -69,6 +69,23 @@ func ProfileListIndex(args *ProfileListIndexArgs) (int, error) {
 		}
 	}
 
+	inStore, err := args.Input.IsInBinaryStore()
+	if err != nil {
+		return -1, err
+	}
+	if inStore {
+		pathInStore, err := args.Input.PathInBinaryStore()
+		if err != nil {
+			return -1, err
+		}
+		for _, item := range list {
+			if pathInStore == item.nixStorePath {
+				return item.index, nil
+			}
+		}
+	}
+	// else: fallback to checking if the Input matches an item's unlockedReference
+
 	// This is an optimization for happy path. A resolved devbox package
 	// should match the unlockedReference of an existing profile item.
 	ref, err := args.Input.NormalizedDevboxPackageReference()
@@ -191,7 +208,13 @@ type ProfileInstallArgs struct {
 // ProfileInstall calls nix profile install with default profile
 func ProfileInstall(args *ProfileInstallArgs) error {
 	input := devpkg.PackageFromString(args.Package, args.Lockfile)
-	if nix.IsGithubNixpkgsURL(input.URLForFlakeInput()) {
+
+	isInBinaryStore, err := input.IsInBinaryStore()
+	if err != nil {
+		return err
+	}
+
+	if !isInBinaryStore && nix.IsGithubNixpkgsURL(input.URLForFlakeInput()) {
 		if err := nix.EnsureNixpkgsPrefetched(args.Writer, input.HashFromNixPkgsURL()); err != nil {
 			return err
 		}
@@ -204,12 +227,12 @@ func ProfileInstall(args *ProfileInstallArgs) error {
 		fmt.Fprintf(args.Writer, "%s\n", stepMsg)
 	}
 
-	urlForInstall, err := input.URLForInstall()
+	installable, err := installableForPackage(input)
 	if err != nil {
 		return err
 	}
 
-	err = nix.ProfileInstall(args.Writer, args.ProfilePath, urlForInstall)
+	err = nix.ProfileInstall(args.Writer, args.ProfilePath, installable)
 	if err != nil {
 		fmt.Fprintf(args.Writer, "%s: ", stepMsg)
 		color.New(color.FgRed).Fprintf(args.Writer, "Fail\n")
@@ -234,3 +257,28 @@ func ProfileRemoveItems(profilePath string, items []*NixProfileListItem) error {
 	}
 	return nix.ProfileRemove(profilePath, indexes)
 }
+
+// installableForPackage determines how nix profile should install this package.
+// Keeping in `nixprofile` package since its specific to how nix profile works,
+// rather than a general property of devpkg.Package
+func installableForPackage(pkg *devpkg.Package) (string, error) {
+	isInBinaryStore, err := pkg.IsInBinaryStore()
+	if err != nil {
+		return "", err
+	}
+
+	if isInBinaryStore {
+		// TODO savil: change to ContentAddressablePath when that is implemented
+		installable, err := pkg.PathInBinaryStore()
+		if err != nil {
+			return "", err
+		}
+		return installable, nil
+	}
+
+	installable, err := pkg.URLForInstall()
+	if err != nil {
+		return "", err
+	}
+	return installable, nil
+}

internal/nix/profiles.go

@@ -48,7 +48,7 @@ func ProfileList(writer io.Writer, profilePath string) ([]string, error) {
 	return lines, nil
 }
 
-func ProfileInstall(writer io.Writer, profilePath string, urlForInstall string) error {
+func ProfileInstall(writer io.Writer, profilePath string, installable string) error {
 
 	cmd := command(
 		"profile", "install",
@@ -58,7 +58,7 @@ func ProfileInstall(writer io.Writer, profilePath string, urlForInstall string)
 		// Note that this is not really the priority we care about, since we
 		// use the flake.nix to specify the priority.
 		"--priority", nextPriority(profilePath),
-		urlForInstall,
+		installable,
 	)
 	cmd.Env = allowUnfreeEnv()