generated code position comments should use relative file paths

[scoder]

Currently, Cython generates absolute file paths into the source code position comments, e.g.

/* "/[...]/cython/cython-devel/Cython/Compiler/Code.pxd":62
 * 
 *     @cython.locals(intern=bint, is_str=bint, is_unicode=bint)
 *     cpdef get_py_string_const(self, encoding, identifier=*, is_str=*)             # <<<<<<<<<<<<<<
 * 
 * ## cdef class PyStringConst:
 */

It would be better to use file paths relative to the project root, or even the package root of the project (thus printing basically the module's package path only).

This would reduce the size of the generated file, avoid leaking information about the file system layout of the author and (to a certain extent) better support diffs when comparing generated C files from different project checkouts (e.g. different Cython versions).

[scoder]

Fix is here:

 https://github.com/cython/cython/commit/c6be9c54a42995f82fcfdf5bdbf0d1e3d5f4a88e

 https://github.com/cython/cython/commit/ed53fdd918ad67ef0d1689ddec0b389f3d8268d6