Skylib skylark library (#64)

* Fix provider signature.

https://docs.bazel.build/versions/master/skylark/lib/globals.html#provid
er

* Generate docs only for `srcs`.

Fixes #63.

* Use Skylib's `skylark_library`.

Import a slightly modified `skylark_library` which also allows for test dependencies, and update `skydoc_library` to use the alternate provider.

* review feedback

* don't union depsets

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index 529f895..3dbce63 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -5,6 +5,12 @@
     remote = "https://github.com/bazelbuild/rules_sass.git",
     tag = "0.0.3",
 )
+git_repository(
+    name = "bazel_skylib",
+    remote = "https://github.com/bazelbuild/bazel-skylib.git",
+    tag = "0.2.0",
+)
+
 load("@io_bazel_rules_sass//sass:sass.bzl", "sass_repositories")
 sass_repositories()