Now commands can depend on `{boot.bindings.<binding name>}` placeholders
which are satisfied by boot bindings commands that are run exactly once.

This is used in the Python example to produce a scie-pants Pants binary
that prepares a fully pre-compiled Pants venv that runs at the full
speed of a native Pants venv. The boot binding command that creates the
venv also uses a private PEX_ROOT and cleans this up in addition to the
Pants PEX used to build the venv. The result is an ~/.nce with just the
CPython interpreter and a venv that symlinks to it. There is no wasted
speed or space as compared to a traditional Pants Python venv (save for
the size of the scie binary itself which may be addressed by a-scie#9 or a-scie#19).

Closes a-scie#7
Closes a-scie#20

Now the intracies of recognizing a scie tote are self contained in
the Context.

…der.