Let rules_jsonnet use its own provider (#172)

Right now it uses struct() to declare information that's yielded, but
this makes it annoying to use, especially when using it in combination
with other rules that do use providers.

jsonnet/jsonnet.bzl

@@ -71,14 +71,16 @@ def _setup_deps(deps):
     transitive_sources = []
     imports = []
     for dep in deps:
-        transitive_sources.append(dep.transitive_jsonnet_files)
-        imports.append(dep.imports)
+        transitive_sources.append(dep[JsonnetLibraryInfo].transitive_jsonnet_files)
+        imports.append(dep[JsonnetLibraryInfo].imports)
 
     return struct(
         imports = depset(transitive = imports),
         transitive_sources = depset(transitive = transitive_sources, order = "postorder"),
     )
 
+JsonnetLibraryInfo = provider()
+
 def _jsonnet_library_impl(ctx):
     """Implementation of the jsonnet_library rule."""
     depinfo = _setup_deps(ctx.attr.deps)
@@ -88,15 +90,19 @@ def _jsonnet_library_impl(ctx):
         transitive = [dep.data_runfiles.files for dep in ctx.attr.deps],
     )
 
-    return struct(
-        files = depset(),
-        imports = imports,
-        runfiles = ctx.runfiles(
-            transitive_files = transitive_data,
-            collect_data = True,
+    return [
+        DefaultInfo(
+            files = depset(),
+            runfiles = ctx.runfiles(
+                transitive_files = transitive_data,
+                collect_data = True,
+            ),
         ),
-        transitive_jsonnet_files = sources,
-    )
+        JsonnetLibraryInfo(
+            imports = imports,
+            transitive_jsonnet_files = sources,
+        ),
+    ]
 
 def _jsonnet_toolchain(ctx):
     return struct(
@@ -433,13 +439,13 @@ def _jsonnet_to_json_test_impl(ctx):
         stamp_inputs
     )
 
-    return struct(
+    return [DefaultInfo(
         runfiles = ctx.runfiles(
             files = test_inputs,
             transitive_files = transitive_data,
             collect_data = True,
         ),
-    )
+    )]
 
 _jsonnet_common_attrs = {
     "data": attr.label_list(
@@ -457,7 +463,7 @@ _jsonnet_common_attrs = {
     ),
     "deps": attr.label_list(
         doc = "List of targets that are required by the `srcs` Jsonnet files.",
-        providers = ["transitive_jsonnet_files"],
+        providers = [JsonnetLibraryInfo],
         allow_files = False,
     ),
 }