defs.bzl relies on transitive loads

[laurentlb]

For context, see:

• https://docs.bazel.build/versions/master/skylark/backward-compatibility.html#disallow-transitive-loads
• incompatible_no_transitive_loads: Forbid macros loaded transitively bazel#5636

A solution is to re-export the symbols explicitly (the main benefit is that users won't need to update their code):

load("//closure/compiler:closure_js_aspect.bzl", "_closure_js_aspect")
load(...)

closure_js_aspect = _closure_js_aspect
...