Everything appears to be functional. I assumed that two functions that
were called the same thing with approximately the same contents had the
same behavior... not a safe assumption to make. That function is now
reimplemented, among with several other small fixes. I also added a call
to `pkgar list` in the tests to make sure it doesn't panic or do
something else equally dumb.

I also added a dependency on UserFacingError... I'm not sure how much
value it will add, but I can see it being more useful as pkgar matures
and good error reporting becomes more critical. Many improvements on
that front soon I hope.

In terms of the refactoring that happened last commit: There are some
critical issues with implementing PackageSrc as a trait, namely needing
to call read_at (which should be considered expensive) in order to read
the header, something that most of the functions have to do. read_at
could conceiveably cross network boundaries in the future, so it's
important not to call it unessesarily. More design work needs to be done
there.