[rubygems/rubygems] Add a warning in an edge case of using `gemspec` DSL

If a Gemfile duplicates a development dependency also defined in a local
gemspec with a different requirement, the requirement in the local
gemspec will be silently ignored.

This surprised me.

I think we should either:

* Make sure both requirements are considered, like it happens for
  runtime dependencies (I added a spec to illustrate the current behavior
  here).

* Add a warning that the requirement in the gemspec will be ignored.

I think the former is slightly preferable, but it may cause some
bundle's that previously resolve to no longer resolver.

I went with the latter but the more I think about it, the more this
seems like it should behave like the former.

https://github.com/rubygems/rubygems/commit/ad6843972f

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