Windows executable paths can have spaces in them, which was leading to
errors when constructing Executable objects: the parser was intended
to handle cases where users could provide an executable along with one
or more space-delimited arguments.
* Executable now assumes that it is constructed with a string argument
that represents the path to the executable, but no additional arguments.
* Invocations of Executable.__init__ that depended on this have been
updated (this includes the core, tests, and one instance of builtin
repository package).
* The error handling for failed invocations of Executable.__call__ now
includes a check for whether the executable name/path contains a
space, to help users debug cases where they (now incorrectly)
concatenate the path and the arguments.
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