is there a particular reason for PyOS_FSPath to live in posixmodule.c

since Objects/unicodeobject.c uses this function, this makes it not possible to compile Python without a posixmodule.

this makes it difficult to compile a 'core' python on a new platform, since implementing the posixmodule on a new platform is not trivial.

also the documentation on porting mentions that one should first port python without a posixmodule.