Remove 'already initialized constant' warnings

What does this MR do?

Get rid of already initialized constant warnings, such as

lib/gitlab.rb:40: warning: already initialized constant Gitlab::COM_URL
lib/gitlab.rb:40: warning: previous definition of COM_URL was here
lib/gitlab.rb:41: warning: already initialized constant Gitlab::APP_DIRS_PATTERN
lib/gitlab.rb:41: warning: previous definition of APP_DIRS_PATTERN was here
lib/gitlab.rb:42: warning: already initialized constant Gitlab::SUBDOMAIN_REGEX
lib/gitlab.rb:42: warning: previous definition of SUBDOMAIN_REGEX was here
lib/gitlab.rb:43: warning: already initialized constant Gitlab::VERSION
lib/gitlab.rb:43: warning: previous definition of VERSION was here
lib/gitlab.rb:44: warning: already initialized constant Gitlab::INSTALLATION_TYPE
lib/gitlab.rb:44: warning: previous definition of INSTALLATION_TYPE was here

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content