toplevel build system continued

- dont stage python-sage
- install helper fixes
- distcheck fixes
cython: add -MF switch to write dependency files (#14728)

... and -M -F -D -P
sagelib: build system (#14807)

this implements a plain ./configure && make interface for the parts
located in /src
sage-ext: build-system
sage-doc: move files
sage-doc: build system

- SAGE_DOC_SRC
- quick hacks
sage-bin: move files (make templates)
sage-bin: sage-build
sage-bin: sage-spkg.in
sage-bin: sage.in

- config files to SAGE_ETC=@sysconfdir@
sage-bin build system

- add environment for build
- SAGE64 maybe incomplete
- remove sage-arch-env
sage-bin: simplify sage-sdist
sage-bin: typo
sage-bin: env

- remove MPLCONFIGDIR hack
sage-bin: continued
python-sage: check whether gprc.expect is available (#14749)

(otherwise subsequent tests will timeout)
python-sage SAGE_ETC (#14750)

- gp.py
- qepcad.py
python-sage: fix relative paths in include statements
python-sage: move files around, turn into templates

(this is to help git keeping track)
python-sage more explicit import/include paths

intended to work with -I<parent_directory_of_'sage'>