[rubygems/rubygems] Restore `gem_dir` as an instance variable accessorHEADmaster

I suspect someone could be setting this instance variable, and the
previous changes made that no longer effective.

Also I implemented a previous TOOD in `full_gem_path` the other way
around:

> # TODO: This is a heavily used method by gems, so we'll need
> # to aleast just alias it to #gem_dir rather than remove it.

I made `gem_dir` an alias of `full_gem_path` rather than the opposite.

This alternative change keeps both methods symmetric without deprecating
either of them for now.

https://github.com/rubygems/rubygems/commit/28983973a3

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