Add pypy2 component

Building PyPy from source is a nightmare and the shipped top-level
Makefile is a joke:
- nothing configurable at top-level to do the 2 common operations:
  build & install;
- the main binary searches for the library at hardcoded and
  undocumented places relative to where it is installed;
- no option to exclude specific components ? (here, _tkinter);
- lack documentation to specify location of required libraries;
- nothing out-of-the-box to build with several cores (it's even
  strange that in 2020, the biggest part is single threaded and
  it can't be resumed; I'm curious to know how developers work).

I found many answers in the Debian packaging.

This commit creates a Makefile file for 2 reasons:
- slapos.recipe.cmmi (forked from hexagonit.recipe.cmmi)
  does not have options to specify a shell snippet that is
  executed only during the build step
- by splitting the build in several steps, it makes debugging
  easier when it does not have to rebuild from the beginning

The drawback is that:
- make recipes need to be indented with tabs,
  in addition to the buildout indent of 2 spaces;
- $ escaping is tricky.

component/pypy/buildout.cfg

+[buildout]
+extends =
+  ../bzip2/buildout.cfg
+  ../gdbm/buildout.cfg
+  ../libexpat/buildout.cfg
+  ../libffi/buildout.cfg
+  ../ncurses/buildout.cfg
+  ../openssl/buildout.cfg
+  ../sqlite3/buildout.cfg
+  ../zlib/buildout.cfg
+
+[pycparser-shared]
+# XXX:
+recipe = slapos.recipe.cmmi
+shared = true
+url = https://github.com/eliben/pycparser/archive/release_v2.20.tar.gz
+md5sum = a5d9ea5350a8edb8239af73913ea2858
+configure-command = :
+make-binary =
+make-targets = python setup.py install --install-lib @@LOCATION@@
+
+[pypy2]
+recipe = slapos.recipe.cmmi
+shared = true
+url = https://bitbucket.org/pypy/pypy/downloads/pypy2.7-v7.3.1-src.tar.bz2
+md5sum = 7608bd58940ffc5403632c2c786d83bb
+configure-command =
+  sed -i '/"_tkinter":/s/^/#/' lib_pypy/tools/build_cffi_imports.py
+  cat <<EOF > Makefile
+  PREFIX = @@LOCATION@@
+  export PYPY_USESSION_BASENAME=slapos
+  export TMPDIR=\$(realpath ..)
+  all: pypy/goal/pypy-c lib_pypy/tools/build_cffi_imports.py
+  	\$^
+  c_src_dir:
+  	cd pypy/goal && PYTHONPATH=${pycparser-shared:location} $${PYTHON:-python2}  ../../rpython/bin/rpython --batch --source --opt=jit --shared targetpypystandalone
+  	ln -s ../usession-\$\$PYPY_USESSION_BASENAME-\$\$USER/testing_1 \$@
+  pypy/goal/pypy-c: c_src_dir
+  	\$(MAKE) -C \$<
+  	mv \$</libpypy-c.so \$</pypy-c pypy/goal
+  	touch \$@
+  install:
+  	mkdir -p \$(PREFIX)/bin \$(PREFIX)/include
+  	find lib_pypy lib-python/2.7 -type d '(' '(' -name __pycache__ -o -name _tkinter -o -name test -o -name tests ')' -prune -o -print ')' \
+  	|while read d; do mkdir -p \$(PREFIX)/\$\$d && find \$\$d -maxdepth 1 -type f ! -name '*.o' ! -name '*.c' |xargs -r cp -t \$(PREFIX)/\$\$d; done
+  	d=lib-python/2.7/test && mkdir -p \$(PREFIX)/\$\$d && for x in __init__ pystone regrtest test_support; do echo \$\$d/\$\$x.py; done |xargs -r cp -t \$(PREFIX)/\$\$d
+  	cd lib-python && cp conftest.py stdlib-version.* \$(PREFIX)/lib-python
+  	cp -r include/pypy_*.h include/PyPy.h pypy/module/cpyext/include/* pypy/module/cpyext/parse/* \$(PREFIX)/include
+  	cd pypy/goal && cp libpypy-c.so \$(PREFIX)/bin && cp pypy-c \$(PREFIX)/bin/pypy
+environment =
+  C_INCLUDE_PATH=${bzip2:location}/include:${gdbm:location}/include:${libexpat:location}/include:${ncurses:location}/include:${ncurses:location}/include/ncursesw:${openssl:location}/include:${sqlite3:location}/include:${zlib:location}/include
+  LDFLAGS=-L${bzip2:location}/lib -L${gdbm:location}/lib -L${libexpat:location}/lib -L${libffi:location}/lib -L${ncurses:location}/lib -L${openssl:location}/lib -L${sqlite3:location}/lib -L${zlib:location}/lib -Wl,-rpath=${bzip2:location}/lib -Wl,-rpath=${gdbm:location}/lib -Wl,-rpath=${libexpat:location}/lib -Wl,-rpath=${libffi:location}/lib -Wl,-rpath=${ncurses:location}/lib -Wl,-rpath=${openssl:location}/lib -Wl,-rpath=${sqlite3:location}/lib -Wl,-rpath=${zlib:location}/lib
+  PKG_CONFIG_PATH=${libffi:location}/lib/pkgconfig