Global users can't recover their login from a subwiki

Steps to reproduce:

1. Create a subwiki with option "Both global and local users are available in the wiki"
2. In a private window, access to the subwiki and click on "Log-In"
   (http://localhost:8080/xwiki/wiki/subwiki/login/XWiki/XWikiLogin)
3. On the Log-in page, click on "Forgot your username?"
4. Put the email address of a Global user.
5. Even if this user is registered in the farm and has access to the wiki, the following message is displayed "No account is registered using this email address."

It causes issues in a farm with several subwikis, and users sometimes forgetting their login. When they try to connect to a subwiki and want to recover their login, they don't manage to retrieve it.

Possible workaround: If the email address is known as a global user email address, redirect to the main wiki login page.
(http://localhost:8080/xwiki/bin/login/XWiki/XWikiLogin)

Relates to XWIKI-15682