The parser sees this as a positional argument, but when requesting completions
this could be either a filename that starts with a hyphen, or it could be a
flag. This expands the completion engine's interface to return a vec of possible
completion locations instead of an optional one, because we want to show all
possibilities instead of assuming one or the either.
The completion engine maps completion locations to spans on a line, which
indicate whther to complete a command name, flag name, argument, and so on.
Initial implementation is simplistic, with some rough edges, since it relies
heavily on the parser's interpretation. For example
du -
if asking for completions, `-` is considered a positional argument by the
parser, but the user is likely looking for a flag. These scenarios will be
addressed in a series of progressive enhancements to the engine.
