The default location of osm2pgsql and the postgresql module
is decided at compile/installation time and is not necessarily
in the project directory.
With this change it is now possible to have a project directory
that is completely separate from the build directory.
As we can't refer to the project root dir in the module path, the
module path may now also be a relative directory which is then
taken as being relative to the project root path.
Moves the checkModulePresence() function into the Setup class, so
that it can work on the computed absolute module path.
