re2: fix reporting of availability in `hg debuginstall` We add and use an official API to check is re2 is available. This prevent the bug previously in place were debuginstall was considering confusing `None` (i.e. non-initialized) and `False` (i.e. unavailable).

re: make _checkre2 a static method There is no need to it to be a method, so we make it at least a static method to make it easier to call.