Actions with non-trivial cwd: add empty input tree, if required

While our local action execution implicitly creates the specified
cwd with the first output file or directory, this behaviour is
not mandated by the remote-execution protocol. There, an action
definition has to ensure that cwd is a directory implied by the
input files. Achieve this, by adding an empty input directory
at cwd if this can be done without creating tree conflicts.

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