WIP pactivate: proposed fix for Debian Python 3.10 pkg w/o distutils

*** Tests (all manual so far):

                  ----- Debian package ------
    Tested on     python3   python3-distutils   result after fix
    ──────────────────────────────────────────────────────────────────────
    Ubuntu 22.04   3.10        NO               problem fixed
    Ubuntu 22.04   3.10        YES              still working
    Debian 11      3.9         YES              still working
    Debian 11      3.9         NO               fails with clear message

*** To-do:

- Discuss what we want to do for testing of the workaround. Currently we
  test only in debian:11, which confirms that the workaround does not
  affect that (where it's not needed) but does _not_ confirm that the
  workaround does work where it is needed (ubuntu:22.04 w/o
  python3-distutils).

*** Bugs in previous code:

- Deb 11 w/o python3-distutils fails to install bootstrap pip, prints a
  message, and should stop, but instead goes on with "Installing
  bootstrap virtualenv".
- Also separate commit to add commented-out "head-of-main" thing below.

*** Changes since last good commit (some need to be moved to other commits)

- pactivate:
  - Add commented-out "head of main" package for virtualenv
    - XXX move to a separate commit
  - add DEB_PYTHON_INSTALL_LAYOUT workaround
- cont-test:
  - remove the debugging
  - XXX keep a check for pactivate failure:
      (. ./$pactivate) || fail "pactivate failed: return code $?"
  - XXX keep large comment about system python 3.10.x

*** Stuff to document

    print(sysconfig.get_default_scheme())

    [2350]: pypa/virtualenv#2350
    [au 1415153]: https://askubuntu.com/a/1415153/354600

    [2415]: https://github.com/pypa/virtualenv/pull/2415/files?diff=unified&w=0

*** Maybe document

    pypa/setuptools#3278

cont-test

@@ -68,17 +68,6 @@ echo '━━━━━ run pactivate clean'
 [[ -d $bsve/virtualenv/ ]] || fail "not a dir: $bsve/virtualenv/"
 $virtualenv/bin/pip --version || fail "pip --version didn't work"
 
-# temp stop and look
-echo
-ls -CF .build/virtualenv/
-fail "XXX Tests temporarily stop here"
-
-echo '━━━━━ XXX debugging test'
-rm -rf .build
-ln -s $(/usr/local/pythonz/bin/pythonz locate 3.10.7) .python
-(. $pactivate)
-$virtualenv/bin/pip --version || fail "pip --version didn't work"
-
 echo '━━━━━ run pactivate installed'
 #   This should do nothing but run the existing virtualenv's activate
 #   It also tests the -q option so we can leave it out (by default)

pactivate

@@ -64,6 +64,8 @@ mkdir -p "$__pa_bootdir"    # also ensures that $__pa_builddir exists
     echo '----- Installing bootstrap virtualenv'
     PYTHONPATH="$__pa_bootdir" "$__pa_bootdir/bin/pip" \
         $__pa_quiet install -t "$__pa_bootdir" virtualenv
+        #   If you want to use "current" virtualenv instead, install:
+        #       git+https://github.com/pypa/virtualenv@main
 }
 
 [[ -d $__pa_builddir/virtualenv/ ]] || {
@@ -83,6 +85,7 @@ mkdir -p "$__pa_bootdir"    # also ensures that $__pa_builddir exists
         echo
     fi
     echo -n "Version: " && "$__pa_python" --version
+    DEB_PYTHON_INSTALL_LAYOUT='deb' \
     PYTHONPATH="$__pa_bootdir" "$__pa_python_deref" -s -m virtualenv \
         $__pa_quiet --prompt $(dirname "$__pa_basedir") \
         "$__pa_builddir/virtualenv/" \