Fix config#check to use loaded_path in warning

[chrishulton]

Due to a configuration refactor, the #path variable is no longer
available in config#check, which causes rubocop to crash with
NameError: undefined local variable or method path if the deprecation
check yields a deprecation warning.

This replaces the warning to use the instance variable #loaded_path
and adds a spec to cover the behavior.

---

This is a small bug that looks to have been introduced with this commit and is impacting version 0.52.

Without the code change, the spec fails with:

 NameError:
   undefined local variable or method `path' for #<RuboCop::Config:0x007fe191cc9a00>
 # ./lib/rubocop/config.rb:198:in `block in check'
 # ./lib/rubocop/config.rb:288:in `block in deprecation_check'
 # ./lib/rubocop/config.rb:282:in `each'
 # ./lib/rubocop/config.rb:282:in `deprecation_check'
 # ./lib/rubocop/config.rb:197:in `check'
 # ./spec/rubocop/config_spec.rb:533:in `block (4 levels) in <top (required)>'

This is also reproducible by defining the configuration:

AllCops:
  Excludes:
    - 'path'

which results in an exit with the NameError described above.

Please let me know if I should add anything else to this PR! Thanks!

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run rake default or rake parallel. It executes all tests and RuboCop for itself, and generates the documentation.

[volmer]

Thank you! I was experiencing the same problem. I hope the gem gets patched soon 🙏

[bbatsov]

The change looks good to me. I'll leave it to @jonas054 to review it as well before merging this patch , though, as he worked on the change that caused the bug.

[bbatsov]

P.S. This needs a changelog entry.

[chrishulton]

Thanks, changelog added.

[jonas054]

Yes, that was my fault. I missed that variable when I moved the method. The change looks good! 👍