For posix-ish systems, we previously checked that a path was relative
by simpy checking if it included a '/' character. This meant that you
could not, for example, do
```
$ cat foo.substitutions
file rel/to/bar.template {
}
$ msi -I /some/path foo.substitutions
```
where our template file is located at `/some/path/rel/to/bar.template`.
Note that relateive paths work differently on Windows, so we carve out
an exception there.
