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Compose a Process out of multiple sequential subprocesses (#23400)

It is sometimes useful to run multiple processes in a single
Process invocation. 

For example, when creating a pex from a venv repository,
we need to ensure the venv repository exists, and the 
only reliable way to do so in the presence of remote caching/
remote execution is to run the venv creation process 
in the same Process as the pex invocation.

This change introduces a bit of useful plumbing to make
this easy: A `CompositeProcess` holds multiple
`Subprocess` instances, and invokes them sequentially
via a bash command.

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