Refactor identifier parsing

What does this MR do?

This MR changes the parsing of GlRepository::Identifiers for clarity. It:

• introduces subclasses to deal with the distinction between two-part and three-part formats
• does parsing in parse instead of new
• adds a new ArgumentError subclass

It has no functional changes, and is aimed at code clarity and maintainability.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry ~backstage change, so no necessary
• Documentation (if required) Not needed
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

This MR is not intended to introduce functional changes. We have good test coverage here, and that is not changed.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

/label ~backstage