sagelib: spkg, draft

this will be required by a proper toplevel build system
sage-bin: cleanup env

- adapt env
- remove sage-arch-env
- remove SAGE_ROOT hacks
- remove ECLDIR (ecl takes care of it)
sage-bin adapt various scripts

- sage-build
- sage-spkg.in
- sage-sdist
sage-bin: sage.in

- config files to @SAGE_ETC@
- cleanup
- PIPESTATUS_CMD
sage-bin: sage-build
toplevel transition: cleanup bin/sage-env

runtime paths will be taken care of by sagelib/spkg-install
sagenb: postponing; no idea how to workaround #14840
python-sage more explicit import/include paths (unneeded?)

intended to work with -I<parent_directory_of_'sage'>
python-sage: simplify include directives, lrcalc, fplll?

- lrcalc will not work without -I/path/to/include/lrcalc anyway
- fplll will not work without -I/path/to/include/fplll anyway
python-sage: hack sage_getfile (still needed?)

sage_getfile is expected to return the absolute path to the *source*
file. inspect.getabsfile returns the path to the compiled object, if
it's not in the same direcory.
python-sage: comment on header hack
python-sage: make libgap obsolete (I)

import various stuff + to be modified vanilla gap source
python-sage: make libgap obsolete, II (patches against gap upstream)
python-sage: fix module import path

(this is apparently required in sagelib?)
sage-python move checks around
sage-bin: rearrange config files

some config files need to be part of sagelib, so check works in bare
builds. for now. alternatively one could
- get rid of those files (e.g. sage-maxima.lisp) or
- distribute them with packages sagelib depends on
TODO update