[Fix #7576] Fix an error for Gemspec/OrderedDependencies

[koic]

Fixes #7576.

This PR fixes the following error for Gemspec/OrderedDependencies when using a local variable in an argument of dependent gem.

% cat example.gemspec
Gem::Specification.new do |s|
  %w[foo bar].each { |dep| s.add_dependency dep }
  s.add_dependency 'baz'
end

% rubocop --only Gemspec/OrderedDependencies example.gemspec -d
(nsip)

An error occurred while Gemspec/OrderedDependencies cop was inspecting
/Users/koic/src/github.com/koic/rubocop-issues/7576/example.gemspec.
undefined method `str_type?' for nil:NilClass

Since it is difficult to trace variables and list gem names, this PR will skip them.

And add_dependency, add_runtime_dependency, and add_development_dependency accept a symbol.

e.g. add_dependency(:foo)

The original implementation raises the same error if a symbol is passed as an argument. This PR also solves that problem.

The issue for add_dependency and abc to accept symbols will open as separate a PR.

---

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[bbatsov]

Good fix! I think we should probably add some cop that discourages this practice, though, as gemspecs are much more readable when written in a purely declarative fashion.