[Bro-Dev] script loading changes

[Jonathan Siwek]

> We need to bring the current prefix mechanism back to live anyway
> (and with the new load order I think), and this would be quite easy to add
> on top of that if I'm not missing anything.

In my mind I saw the new implementation of @prefixes as independent of that
"@add x when y" idea.  There's some similarities in how they work, but they
don't depend on each other.

> So why don't we give it a try

For now I'll attempt these changes:

- allow relative paths in @load's, e.g. "@load ./software"
- new @prefixes implementation

I'll put these off due to contention:

- allowing '.' vs. '/' in package names
- "@add x when y"