Move plugin dependency_overrides from separate file to pubspec.yaml

[osa1]

It's more common to have the overrides in the main pubspec.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:protobuf	4.0.1-wip	WIP (no publish necessary)
package:protoc_plugin	22.2.0	ready to publish	protoc_plugin-v22.2.0

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

[devoncarew]

lgtm

We can also switch to using the newer pub workspace support. There wouldn't be any downsides for this repo and we could remove the overrides completely.

[osa1]

We can also switch to using the newer pub workspace support. There wouldn't be any downsides for this repo and we could remove the overrides completely.

Makes sense, done in #1001.