Move `Project#pages_lookup_path` to `Pages::VirtualDomain`

What does this MR do and why?

Move Project#pages_lookup_path to Pages::VirtualDomain

This is a small iteration step to solve #328715 (closed).

Related to: #328715 (closed).

Changelog: other

Why not fix #328715 (closed) now?

I'm working on GitLab Pages Multiple Deployments (&10914), which touches the same problem, so instead of fixing it now, I'm doing this small win to reduce the footprint of the final solution.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.