Add a public package for xpkg specification values

Signed-off-by: Jason Tang <jason@upbound.io>

pkg/xpkg/spec.go

@@ -0,0 +1,61 @@
+/*
+Copyright 2025 The Crossplane Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+	http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package xpkg contains constants for values defined by Crossplane's xpkg specification.
+package xpkg
+
+import (
+	"os"
+)
+
+const (
+	// MetaFile is the name of a Crossplane package metadata file.
+	MetaFile string = "crossplane.yaml"
+
+	// StreamFile is the name of the file in a Crossplane package image that
+	// contains its YAML stream.
+	StreamFile string = "package.yaml"
+
+	// StreamFileMode determines the permissions on the stream file.
+	StreamFileMode os.FileMode = 0o644
+
+	// XpkgExtension is the extension for compiled Crossplane packages.
+	XpkgExtension string = ".xpkg"
+
+	// XpkgMatchPattern is the match pattern for identifying compiled Crossplane packages.
+	XpkgMatchPattern string = "*" + XpkgExtension
+
+	// XpkgExamplesFile is the name of the file in a Crossplane package image
+	// that contains the examples YAML stream.
+	XpkgExamplesFile string = ".up/examples.yaml"
+
+	// AnnotationKey is the key value for xpkg annotations.
+	AnnotationKey string = "io.crossplane.xpkg"
+
+	// PackageAnnotation is the annotation value used for the package.yaml
+	// layer.
+	PackageAnnotation string = "base"
+
+	// ExamplesAnnotation is the annotation value used for the examples.yaml
+	// layer.
+	ExamplesAnnotation string = "upbound"
+
+	// MetaAPIGroup is the API group for package metadata.
+	MetaAPIGroup string = "meta.pkg.crossplane.io"
+
+	// MetaAnnotationKey is the key value for xpkg metadata annotations.
+	MetaAnnotationKey string = "meta.crossplane.io"
+)