Add support for `--scie-busybox`.

[jsirois]

This allows you to turn your PEX into a native BusyBox scie by specifying which entry points to expose as BusyBox commands.

[jsirois]

Reviewers, a quickie before I hit the road. I'll be back on 7/25 to add tests.

For example though:

# Build a BusyBox PEX scie with 3 entry-points: 1 console script and 2 module eps from the stdlib:
:; python -mpex cowsay --scie lazy --scie-busybox cowsay,venv:venv,http-server:http.server -obusy-box

# Since the exe name does not match any ep, you get BusyBox help:
:; ./busy-box
Error: Could not determine which command to run.

Please select from the following boot commands:

cowsay
http-server
venv

You can select a boot command by setting the SCIE_BOOT environment variable or else by passing it as the 1st argument.

# Now use the BusyBox:
:; ./busy-box http-server -h
usage: server.py [-h] [--cgi] [--bind ADDRESS] [--directory DIRECTORY] [port]

positional arguments:
  port                  specify alternate port (default: 8000)

options:
  -h, --help            show this help message and exit
  --cgi                 run as CGI server
  --bind ADDRESS, -b ADDRESS
                        specify alternate bind address (default: all interfaces)
  --directory DIRECTORY, -d DIRECTORY
                        specify alternate directory (default: current directory)

# Now install a link farm for more convenient use (if the install directory is on your PATH anyhow):
:; SCIE=install ./busy-box -s /tmp/bin
:; ls -l /tmp/bin/
total 0
lrwxrwxrwx 1 jsirois jsirois 39 Jul 17 15:15 cowsay -> /home/jsirois/dev/pex-tool/pex/busy-box
lrwxrwxrwx 1 jsirois jsirois 39 Jul 17 15:15 http-server -> /home/jsirois/dev/pex-tool/pex/busy-box
lrwxrwxrwx 1 jsirois jsirois 39 Jul 17 15:15 venv -> /home/jsirois/dev/pex-tool/pex/busy-box

:; /tmp/bin/venv -h
usage: venv [-h] [--system-site-packages] [--symlinks | --copies] [--clear] [--upgrade] [--without-pip] [--prompt PROMPT] [--upgrade-deps] ENV_DIR [ENV_DIR ...]

Creates virtual Python environments in one or more target directories.

positional arguments:
  ENV_DIR               A directory to create the environment in.

options:
  -h, --help            show this help message and exit
  --system-site-packages
                        Give the virtual environment access to the system site-packages dir.
  --symlinks            Try to use symlinks rather than copies, when symlinks are not the default for the platform.
  --copies              Try to use copies rather than symlinks, even when symlinks are the default for the platform.
  --clear               Delete the contents of the environment directory if it already exists, before environment creation.
  --upgrade             Upgrade the environment directory to use this version of Python, assuming Python has been upgraded in-place.
  --without-pip         Skips installing or upgrading pip in the virtual environment (pip is bootstrapped by default)
  --prompt PROMPT       Provides an alternative prompt prefix for this environment.
  --upgrade-deps        Upgrade core dependencies: pip setuptools to the latest version in PyPI

Once an environment has been created, you may wish to activate it, e.g. by sourcing an activate script in its bin directory.

# And always end on a Moo!:
:; mv busy-box cowsay
./cowsay -t Moo!
  ____
| Moo! |
  ====
    \
     \
       ^__^
       (oo)\_______
       (__)\       )\/\
           ||----w |
           ||     ||

[jsirois]

Alright reviewers, I got to this earlier than expected and I think its good to go for a final review.

[jsirois]

@sureshjoshi I'd like to proceed with this by tomorrow (August 1st) evening. Let me know if you need more time to review; otherwise, I'll assume you're fine with the @ syntax and no explicit wildcards.

[sureshjoshi]

@jsirois I'm unavailable for any in-depth review until probably Tuesday/Wednesday - but I think we've converged on something and the docs make sense to me