Don't rebuild generated files based on file modification time
anymore, the action is now explicit. Replace "make touch"
with "make regen-all".

Changes:

* Remove "make touch", Tools/hg/hgtouch.py and .hgtouch
* Add a new "make regen-all" command to rebuild all generated files
* Add subcommands to only generate specific files:

  - regen-ast: Include/Python-ast.h and Python/Python-ast.c
  - regen-grammar: Include/graminit.h and Python/graminit.c
  - regen-importlib: Python/importlib_external.h and Python/importlib.h
  - regen-opcode: Include/opcode.h
  - regen-opcode-targets: Python/opcode_targets.h
  - regen-typeslots: Objects/typeslots.inc

* Rename PYTHON_FOR_GEN to PYTHON_FOR_REGEN
* pgen is now only built by by "make regen-grammar"
* Add $(srcdir)/ prefix to paths to source files to handle correctly
  compilation outside the source directory

Note: $(PYTHON_FOR_REGEN) is no more used nor needed by "make"
default target building Python.