When checking the validity of cross references with -Tlint,

When checking the validity of cross references with -Tlint,
fall back from database search to file system search
just like man(1) does when looking up manuals.

This is not too expensive because on a system having up-to-date
mandoc.db(5) files, it only prolongs the time needed to check
*invalid* references - and you are supposed to have many of those,
right?  And on a system with missing or invalid mandoc.db(5) files,
spending a bit of time and warning loudly about the real problem
is also better than quickly issuing bogus warnings about cross
references that are actually valid.