Make Lint/DuplicateMethods aware of alias and alias_method

[pocke]

Currently Lint/DuplicateMethods cop does not aware of alias.

For example:

class A
  def foo
  end
  def bar
  end

  # foo is redefined, but the cop does not detect it.
  alias foo bar
end

This change fixes the problem.

---

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).
• Used the same coding conventions as the rest of the project.
• 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.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[Drenmi]

Good catch! 🙂