[CLEANUP] Mercurial-related data
[REORG] Final fix of file locations
[CLEANUP] Unused files
[CLEANUP] Add gitignores
(FIXUP) make sage-env and basic startup not depend on the spkg directory
(FIXUP) new sage-build script
(FIXUP) move build/install to git layout
(FIXUP) update prereq-install
(FIXUP) build/deps: core repositories

The core repositories are no longer spkgs, so their entries
needed rewriting in deps. Also, the root repository is no longer
installed, so nothing can depend on it.
(FIXUP) sage-spkg: support legacy spkgs
(FIXUP) sage-spkg: support local scripts
(FIXUP) update sage-starts
(FIXUP) copy csage, including headers
(FIXUP) sage: fix pipestatus location
(FIXUP) correct path for docbuilder
(FIXUP) do not create spkg/build anymore
(FIXUP) ntl: move libtool
(FIXUP) singular: move shared
(FIXUP) sagenb: install into site-packages
(FIXUP) remove mecurial dependency