This adds the `shouldUseExecutableName` property, allowing the
command name to be derived from the executable's file name.

The property defaults to false, both because subcommands using
it is probably undesirable and to preserve existing behaviour
after updating the package.

This doesn't change test behaviour in any way. Single quotes were escaped
unnecessarily for some tests, these escapes have been removed to make
them consistent and clearer.

Foundation imports were reduced to only the necessary protocols or
types in the main branch. After merging, we need to import
`Foundation.URL` for use in `executableName`.