For consistency with `RustClientCodegenPlugin`. This is a better name,
since the plugin is named `rust-server-codegen`.

This commit also contains other drive-by improvements made while working
on #2302.

Rename `RustCodegenServerPlugin` to `RustServerCodegenPlugin`, for
consistency with `RustClientCodegenPlugin`. This is a better name, since
the plugin is named `rust-server-codegen`.

This commit also renames `PythonCodegenServerPlugin` to
`RustServerCodegenPythonPlugin` for the same reasons.

This commit also contains other drive-by improvements made while working
on #2302.