SearchResultPageIdentity and SearchResultPageIdentityValue were introduced as placeholders for PageIdentity and PageIdentityValues. They can now become aliases.

• Turn SearchResultPageIdentity and SearchResultPageIdentityValue into aliases
• Change Wikibase and PageImages to use the proper types instead of the aliases
• remove the aliases from core.

Note that both types had been marked @unstable on creation, so the aliases can now be removed as soon as the extensions have been updated, without the need to retain backwards compatibility in the next release.