internal/rosa: optionally block packages from early stages

This helps avoid inadvertently increasing bootstrap input count.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/rosa.go

@@ -320,6 +320,8 @@ type PackageAttr struct {
 	// Whether to replace /usr/bin/ with a symlink to /bin/.
 	PopulateUsrBin bool
 
+	// Whether this package is disallowed from bootstrap stages.
+	Std bool
 	// Hint for an early variant of toybox to be used when available.
 	Early bool
 	// Whether to omit output colour environment variables.
@@ -375,6 +377,14 @@ func (t Toolchain) Append(a []pkg.Artifact, handles ...ArtifactH) []pkg.Artifact
 	return a
 }
 
+// A BootstrapError describes a package disallowed from bootstrap stages.
+type BootstrapError string
+
+func (e BootstrapError) Error() string {
+	return "package " + strconv.Quote(string(e)) +
+		" must not be built by bootstrap stages"
+}
+
 // New constructs a [pkg.Artifact] via a build system helper.
 func (t Toolchain) New(
 	name, version string,
@@ -387,6 +397,10 @@ func (t Toolchain) New(
 		attr = new(PackageAttr)
 	}
 
+	if attr.Std && !t.stage.isStd() {
+		panic(BootstrapError(name))
+	}
+
 	if name == "" || version == "" {
 		panic("name must be non-empty")
 	}