Require acorn using its full path "acorn/acorn".

Upstream change for https://bugzilla.mozilla.org/show_bug.cgi?id=967813.

requirejs compatible module systems accept both "acorn" and "acorn/acorn" because they do resolution using package.json. The firefox devtools loader doesn't do this extra resolution (for performance reasons) so it will work with (using hardcoded aliases) either "acorn" or "acorn/acorn" but not both. This change is unrelated to pretty-fast itself, but rather other consumers of acorn that we intend to use in the firefox devtools need the full path (marijnh/tern).