Handle page reference collisions related page naming strategies that are not injective functions

Currently, in import, the name strategy set on the wiki is used to generate page names (apparently since CONFLUENCE-61).
If the current naming strategy can produce the same target name from two different origin names, things will probably not go well.

We should handle the situation by either:

• issuing a warning
• producing a new name to avoid the collision,
  • this needs to be deterministic. Maybe by using the Confluence page id.

Note that all default provided name strategies for XWiki are not injective functions (same output can be obtained for 2 different inputs), but for some the transformation / collision risk is higher than others. Slug name strategy has a higher risk of collision than the character suppression strategy