m4/sage_spkg_collect.m4: Handle 'dependencies_build' files, generate Makefile variables build_deps_...
m4/sage_spkg_collect.m4: Change handling of 'dependencies_check' files, generate Makefile variables check_deps_...
build/make/Makefile.in: Handle build_deps, deps, check_deps separately in package templates
m4/sage_spkg_collect.m4, build/make/Makefile.in: Simplify handling of optional deps
bootstrap, m4/sage_spkg_collect.m4: Get dependencies via 'sage --package dependencies --format=shell'
m4/sage_spkg_collect.m4: Handle wheel packages like normal packages