Remove the resolved TODO from Sources::Local

`Sources::Local#initialize` carries a TODO asking to resolve a relative
source path from the directory of `rbs_collection.yaml`. That was done
three and a half years ago and the comment was left behind.

The TODO arrived in 61dd0e4, a force-push of #854 dated 2022-02-11,
where the constructor was:

    def initialize(source_entry)
      # TODO: resolve relative path from dir of rbs_collection.yaml
      @path = Pathname(source_entry['path'])
    end

There the path really was taken as written and resolved against the
working directory, so the TODO described the code accurately.

86f4ec5, pushed to the same PR on 2023-02-08, added the
`base_directory:` keyword to `Sources.from_config_entry` and changed the
body to `base_directory / path`. `Config#sources` passes
`@config_path.dirname` and `Lockfile.from_lockfile` passes
`lockfile_path.dirname`, so from that point a relative path has been
resolved from the directory holding the configuration -- exactly what
the TODO asked for. Only the code was updated; the comment stayed.

#854 ran from 2021-12-23 to 2023-02-17 and its commits were squashed
into 718997e, so the TODO and the change that answered it landed in one
commit and the leftover was invisible in the history.

Close #3124

#854

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HSq3kKMB8ULhZrnL5urD7y

Author: pocke

lib/rbs/collection/sources/local.rb

@@ -9,7 +9,6 @@ class Local
         attr_reader :path, :full_path
 
         def initialize(path:, base_directory:)
-          # TODO: resolve relative path from dir of rbs_collection.yaml
           @path = Pathname(path)
           @full_path = base_directory / path
         end