This commit won't be quite as useful until I implement RUST_PATH and
until we change `extern mod` to take a general string instead of
an identifier (#5682 and #6407).

With that said, now if you're using rustpkg and a program contains:

extern mod foo;

rustpkg will attempt to search for `foo`, so that you don't have to
provide a -L directory explicitly. In addition, rustpkg will
actually try to build and install `foo`, unless it's already
installed (specifically, I tested that `extern mod extra;` would
not cause it to try to find source for `extra` and compile it
again).

This is as per #5681.

Incidentally, I changed some driver code to infer the link name
from the crate link_meta attributes. If that change isn't ok, say
something. Also, I changed the addl_lib_search_paths field in the
session options to be an @mut ~[Path] so that it can be modified
after expansion but before later phases.