BuildExec translates FOSSA exec structures into standard library exec commands.

Run executes a `Cmd`, retries the specified amount, and checks for command timeout if specified.

Which picks a command out of a list of candidates.

WhichArgs is `Which` but passes multiple arguments to each candidate.

WhichWithResolver is `Which` with a custom resolution strategy.

Cmd represents a single command.

A WhichResolver takes a candidate command and returns whether to choose it.