Previously, we were only doing a binary existence check on Windows. And
in fact, the main point there wasn't binary existence, but ensuring we
didn't accidentally resolve a binary name relative to the CWD, which
could result in executing a program one didn't mean to run.
However, it is useful to be able to check whether a binary exists on any
platform when associating a glob with a binary. If the binary doesn't
exist, then the association can fail eagerly and let some other glob
apply.
Closes#1946
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