nitg: fixed conflicting module error when env var NIT_DIR is specified a...

...nd compiling in /bin

Fixes issue #68:

NIT_DIR env var creates module conflicts when compiling in /nit dir

With NIT_DIR env var setted to /home/ME/nit/, when I compile from /nit/bin like:
    nitg something.nit
I get a compile error saying that there is a module conflict between /home/ME/nit/lib/module1.nit and ../lib/module1.nit.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>