[remove nixpkgs] move installableForPackage to a Package.Installable method (#1258)

## Summary

I didn't feel strongly about where this lived. Moving to
Package.Installable
upon @mikeland73's suggestion in
#1255 (comment)

## How was it tested?

compiles

Author: savil

internal/devpkg/package.go

@@ -145,10 +145,34 @@ func (p *Package) URLForFlakeInput() string {
 	return p.urlWithoutFragment()
 }
 
-// URLForInstall is used during `nix profile install`.
+// Installable for this package. Installable is a nix concept defined here:
+// https://nixos.org/manual/nix/stable/command-ref/new-cli/nix.html#installables
+func (p *Package) Installable() (string, error) {
+	inCache, err := p.IsInBinaryCache()
+	if err != nil {
+		return "", err
+	}
+
+	if inCache {
+		// TODO savil: change to ContentAddressablePath?
+		installable, err := p.InputAddressedPath()
+		if err != nil {
+			return "", err
+		}
+		return installable, nil
+	}
+
+	installable, err := p.urlForInstall()
+	if err != nil {
+		return "", err
+	}
+	return installable, nil
+}
+
+// urlForInstall is used during `nix profile install`.
 // The key difference with URLForFlakeInput is that it has a suffix of
 // `#attributePath`
-func (p *Package) URLForInstall() (string, error) {
+func (p *Package) urlForInstall() (string, error) {
 	if p.isDevboxPackage() {
 		entry, err := p.lockfile.Resolve(p.Raw)
 		if err != nil {

internal/nix/nixprofile/profile.go

@@ -228,7 +228,7 @@ func ProfileInstall(args *ProfileInstallArgs) error {
 		fmt.Fprintf(args.Writer, "%s\n", stepMsg)
 	}
 
-	installable, err := installableForPackage(input)
+	installable, err := input.Installable()
 	if err != nil {
 		return err
 	}
@@ -258,28 +258,3 @@ 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) {
-	inCache, err := pkg.IsInBinaryCache()
-	if err != nil {
-		return "", err
-	}
-
-	if inCache {
-		// TODO savil: change to ContentAddressablePath?
-		installable, err := pkg.InputAddressedPath()
-		if err != nil {
-			return "", err
-		}
-		return installable, nil
-	}
-
-	installable, err := pkg.URLForInstall()
-	if err != nil {
-		return "", err
-	}
-	return installable, nil
-}