sage_setup: Output depfiles

[mkoeppe]

find_python_sources and find_extra_files scan directories in src to find source files. We output these dependencies (on files and directories) in the form of depfiles. This complements the depfiles that cythonize can write out (#34630)

Depends on #32874

Component: build

Issue created by migration from https://trac.sagemath.org/ticket/34631

[mkoeppe]

Dependencies: #32874