Handle relative includes, which escapes .inspector-includes/ better

If I a file is relative to the compiler in an parent directory (ex.: gcc ../../test.cpp) then __FILE__ contains also `..`.
This makes generated headers ending up outside the hidden directory `.inspector-includes`.

Possible hacky solution: Put headers in a sub-sub-sub-sub... directory (`.inspector-includes/a/a/a/a/a/a`) and print a warning if this level get exceeded.