protoc_plugin refactors

[osa1]

• Move static method FileGenerator._getDeclaredMixins to the end of the file
  as a top-level function.

  This is a utility method but it came first in the class and it's quite large
  (~90 lines). Move it to the end so that it will allow top-down reading of the
  class and won't obstruct more important details like fields.

• Refactor import prefix generation: currently if I'm importing container in
  file, I need to do container.importPrefix(context: file). This is
  confusing and it took me a while to figure out what it's doing: It's a method
  on the container type but it modifies the file. Files can import, not
  containers. So files should have import prefixes, not containers.

  Move the method to FileGenerator. The code above now looks like
  file.importPrefix(container).

  Also document the fact that just generating the import prefix for an imported
  thing does not make it imported automatically. This is quite error prone, if
  I call file.importPrefix(container) and use the prefix in e.g.
  EnumGenerator, I need to make sure to update FileGenerator to actually
  generate the import.

  It would be good to refactor this so that just using an prefix is enough to
  import it. But for now this part is not done.

• Minor changes: use spread syntax instead of []..addAll(...) and similar.
  Add one line space between some of the fields.

[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	24.0.0	already published at pub.dev

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

[sigurdm]

https://storage.googleapis.com/dart-specification/DartLangSpecDraft.pdf 19.1 Imports p. 219 specifically defines the p part of import 'uri' as p; as the "prefix", not an "alias"