Take multiple OUT_DIRs and arguments

[Chris Warburton]

We split up AST extraction from exploration, but that requires some way
to tell the exploration where to find the package that defines those
ASTs.

Currently we use an env var `OUT_DIR` internally, and externally
(i.e. when called by a user) we require a command line argument to
specify this.

However, this is not enough. One of the reasons to separate extraction
from exploration is to allow multiple packages to be explored together.
Yet there's no way to pass along multiple paths.

We should replace OUT_DIR with OUT_DIRS, and allow multiple paths to be
given by a user.