Support JavaInfo provider in javadoc_library

Preparation for bazelbuild/bazel#7598.

Closes #75.
Resolves #76.

Testing Done:
- `./build_test.sh`
- Used `javadoc_library` in a project with custom Java rules which only export
  the `JavaInfo` provider.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=254235598

tools/javadoc/javadoc.bzl

@@ -24,7 +24,13 @@ def _android_jar(android_api_level):
 def _javadoc_library(ctx):
     _check_non_empty(ctx.attr.root_packages, "root_packages")
 
-    transitive_deps = [dep.java.transitive_deps for dep in ctx.attr.deps]
+    transitive_deps = []
+    for dep in ctx.attr.deps:
+        if JavaInfo in dep:
+            transitive_deps.append(dep[JavaInfo].transitive_deps)
+        elif hasattr(dep, "java"):
+            transitive_deps.append(dep.java.transitive_deps)
+
     if ctx.attr._android_jar:
         transitive_deps.append(ctx.attr._android_jar.files)