Expose overlay flag (#1361)

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# Description

- new generate flag: `--overlay` 
- fix for recursive copy of overlay files

Fixes #1360 

## Checklist

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- [ ] I have added tests that prove my fix is effective or that my
feature works
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User-facing changes require a CHANGELOG entry.
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- [x] I have updated the
[CHANGELOG-PENDING](https://github.com/pulumi/pulumi/blob/master/CHANGELOG_PENDING.md)
file with my change
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CHANGELOG_PENDING.md

@@ -1,5 +1,6 @@
 ### Improvements
 
 - Plugin: will now automatically use the Gradle executor if build.gradle.kts is present
+- Codegen: support for overlays
 
 ### Bug Fixes

pkg/cmd/pulumi-java-gen/command.go

@@ -76,6 +76,7 @@ See https://www.pulumi.com/docs/guides/pulumi-packages/schema/#language-specific
 	}
 
 	var versionArg, javaSdkVersionArg, schemaArg, outArg, overrideArg, buildArg string
+	var overlays []string
 
 	cmd.Flags().StringVar(&versionArg, "version", "",
 		"default semantic version for the generated package")
@@ -102,6 +103,9 @@ See https://www.pulumi.com/docs/guides/pulumi-packages/schema/#language-specific
 
 `)
 
+	cmd.Flags().StringArrayVar(&overlays, "overlay", nil,
+		"path(s) to folders to mix into the generated code by copying")
+
 	cmd.Run = cmdutil.RunFunc(func(_ *cobra.Command, _ []string) error {
 		rootDir, err := os.Getwd()
 		if err != nil {
@@ -122,6 +126,7 @@ See https://www.pulumi.com/docs/guides/pulumi-packages/schema/#language-specific
 			Version:   version,
 			RootDir:   rootDir,
 			OutputDir: outArg,
+			Overlays:  overlays,
 		}
 
 		if overrideArg != "" {

pkg/cmd/pulumi-java-gen/overlay.go

@@ -24,17 +24,20 @@ func readOverlays(rootDir string, overlays []string) (map[string][]byte, error)
 	result := map[string][]byte{}
 	for _, overlay := range overlays {
 		overlayDir := filepath.Join(rootDir, overlay)
-		err := filepath.WalkDir(overlayDir, func(_ string, entry fs.DirEntry, err error) error {
+		err := filepath.WalkDir(overlayDir, func(path string, entry fs.DirEntry, err error) error {
 			if err != nil {
 				return err
 			}
 			if !entry.IsDir() {
-				sourcePath := filepath.Join(overlayDir, entry.Name())
-				bytes, err := os.ReadFile(sourcePath)
+				relativePath, err := filepath.Rel(overlayDir, path)
 				if err != nil {
 					return err
 				}
-				result[entry.Name()] = bytes
+				bytes, err := os.ReadFile(path)
+				if err != nil {
+					return err
+				}
+				result[relativePath] = bytes
 			}
 			return err
 		})