Add well-known type support

[osa1]

• Add generated Dart for the well-known protos to the package.

• Special case google/protobuf/... imports in the plugin to generate imports
  to the libraries in the protobuf package.

  Note: protoc already special cases these imports, you don't have to specify
  paths to them with -I....

  With this change you also don't have to manually compile them to Dart
  classes.

Fixes #1073.
Fixes #817.
Fixes #780.
Fixes #483.

---

Changes to test files to use internal wkt protos: cl/831301744 (merged)
Output configuration change tested in: cl/831310910

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:protobuf	5.2.0	ready to publish	protobuf-v5.2.0
package:protoc_plugin	25.0.0	ready to publish	protoc_plugin-v25.0.0

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[osa1]

So with this change any breaking change in protoc_plugin becomes a breaking change in protobuf too, because protobuf now exports generated code. This wasn't the case before, we could update protoc_plugin without updating protobuf.

Not sure if this is a big deal in practice, but for example when we merge all of the generated files into one in the future it'll require a major version bump in protobuf with this change.