[Bro-Dev] script loading changes

[Jonathan Siwek]

Robin, Seth, and I were talking about doing the following changes/additions
to the script loading functionality in case anyone has input:

1) When an @load or command line argument relies on BROPATH to search for
   the script/package, directory separator characters (i.e. '/') must be
   replaced with dots ('.').  e.g. "@load frameworks.software".  ".bro"
   file extensions are still optional.

   This makes loading packaged scripts look more similar to other languages
   and better supports the new @prefixes implementation described below.

2) Allow @load to recognize absolute paths or paths relative to the currently-
   loading script.

   e.g. if foo.bro and bar.bro live in the same directory, bar.bro can do
   "@load ./foo".  ".bro" file extension is still optional.

   When a script is loaded relatively, we're still going to have to be able
   to track where it is within BROPATH (if it is at all) in order for change
   (4) described below to work.

   This doesn't really conflict with rule (1) about replacing path separator
   characters with dots when naming scripts-to-load since in this case we're
   not relying on BROPATH, but I wonder if that's not really clear since @load
   is used in both cases, with (2) being implied by arguments that start with
   '.' or '/'.  Maybe we should use a new directive other than @load for this
   case?

3) A new "@add x when y ..." directive that's an alternative to @load, and
   can be used to conditionally load script named 'x' only when the script
   named 'y' has already been loaded.  The evaluation of that condition is
   postponed until after all scripts specified via @load or as command line
   arguments have been loaded.  'x' and 'y' can be named according to either
   (1) or (2) above.

4) The way the @prefixes directive works is going to change.  It currently
   augments future @loads by additionally checking for a prefixed version of
   the argument, meaning @prefixes has to be specified very early on for it to
   work right.  The new implementation of @prefixes will mean: after all
   scripts have been loaded, for each script and for each prefix, search
   BROPATH for a prefixed version of the script in the "dot-syntax" specified
   by (1) and load it if it exists (if the script is a __load__.bro the
   __load__ part is ignored in the canonicalized version of the name such that
   it appears as if it's a "package" once again).

   e.g. doing "@load frameworks.software" and a "@prefixes local" means a file
   in BROPATH named "local.frameworks.software.bro" in BROPATH, will be loaded
   at the end of the script loading process if it exists. ".bro" extension
   still optional.

- Jon