Move danger libs to danger folder

What does this MR do?

This fixes the warnings we see when booting Rails:

/home/git/gitlab/lib/gitlab/danger/title_linting.rb:6: warning: already initialized constant Gitlab::Danger::TitleLinting::DRAFT_REGEX
/home/git/gitlab/lib/gitlab/danger/title_linting.rb:6: warning: previous definition of DRAFT_REGEX was here
/home/git/gitlab/lib/gitlab/danger/weightage.rb:6: warning: already initialized constant Gitlab::Danger::Weightage::CAPACITY_MULTIPLIER
/home/git/gitlab/lib/gitlab/danger/weightage.rb:6: warning: previous definition of CAPACITY_MULTIPLIER was here
/home/git/gitlab/lib/gitlab/danger/weightage.rb:7: warning: already initialized constant Gitlab::Danger::Weightage::BASE_REVIEWER_WEIGHT
/home/git/gitlab/lib/gitlab/danger/weightage.rb:7: warning: previous definition of BASE_REVIEWER_WEIGHT was here
/home/git/gitlab/lib/gitlab/danger/weightage/reviewers.rb:22: warning: already initialized constant Gitlab::Danger::Weightage::Reviewers::DEFAULT_REVIEWER_WEIGHT
/home/git/gitlab/lib/gitlab/danger/weightage/reviewers.rb:22: warning: previous definition of DEFAULT_REVIEWER_WEIGHT was here
/home/git/gitlab/lib/gitlab/danger/weightage/reviewers.rb:23: warning: already initialized constant Gitlab::Danger::Weightage::Reviewers::TRAINTAINER_WEIGHT
/home/git/gitlab/lib/gitlab/danger/weightage/reviewers.rb:23: warning: previous definition of TRAINTAINER_WEIGHT was here

Those warnings are emitted because Rails autoloads these classes and then these classes have explicit requires because these are used in Rake / Danger where there is no autoloader. These cause these classes to be loaded twice when in the Rails context.

This change also means we no longer load these classes when booting or eager-loading our Rails app. We don't need these in that context anyway. This should also save us a tiny bit of Puma memory 🙂

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team