Refactor Gitlab::Regex::BulkImports module

What does this MR do and why?

• Remove the bulk_import_ prefix from the 4 methods in Gitlab::Regex::BulkImports, making sure there's no name conflicts between them and the methods in Gitlab::Regex
• Replace Gitlab::Regex.bulk_import_* with Gitlab::Regex::BulkImports.* across the codebase
• Update the tests and fix any errors that come from the renaming
• Moved the tests related to bulk imports from spec/lib/gitlab/regex_spec.rb into its own separate spec file (spec/lib/gitlab/regex/bulk_imports_spec.rb)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #419767 (closed)