feat(tool/cmd/migrate): migrate Dart library level configuration to librarian configuration (#3765)

Migrate Dart library configuration to librarian configuration.

We use this migration tool to generate a librarian configuration in
google-cloud-dart.

There maybe some edge cases and we may need to adjust the library
configuration accordingly, follow up PRs will resolve library generation
issues.

For #3580

Author: JoeWang1127

internal/config/config.go

@@ -194,7 +194,7 @@ type Library struct {
 	// overrides Default.Transport.
 	Transport string `yaml:"transport,omitempty"`
 
-	// Veneer indicates this library has hand-written code. A veneer may
+	// Veneer indicates this library has handwritten code. A veneer may
 	// contain generated libraries.
 	Veneer bool `yaml:"veneer,omitempty"`
 

internal/config/language.go

@@ -296,6 +296,9 @@ type DartPackage struct {
 	// LibraryPathOverride overrides the library path.
 	LibraryPathOverride string `yaml:"library_path_override,omitempty"`
 
+	// NameOverride overrides the package name
+	NameOverride string `yaml:"name_override,omitempty"`
+
 	// NotForPublication indicates whether this package should not be published.
 	NotForPublication string `yaml:"not_for_publication,omitempty"`
 
@@ -322,4 +325,7 @@ type DartPackage struct {
 
 	// RepositoryURL is the URL to the repository for this package.
 	RepositoryURL string `yaml:"repository_url,omitempty"`
+
+	// TitleOverride overrides the API title.
+	TitleOverride string `yaml:"title_override,omitempty"`
 }