This modification should not change anything logically.

This change simplifies these checks:

• identify an anchor
• identify a relative URI

https://we.phorge.it/T15182

The methods are introduced in PhutilRemarkupRule since that
is the parent class of these children classes:

• PhutilRemarkupDocumentLinkRule
• PhutilRemarkupHyperlinkRule

Ref T15182

• Just check that nothing has been changed logically