better test setup

tests/bzl_library/BUILD

@@ -1,32 +1,32 @@
+load(":bzl_library_consumer.bzl", "bzl_library_consumer")
 load("//:bzl_library.bzl", "bzl_library")
 load("//rules:build_test.bzl", "build_test")
 
-bzl_library(
-    name = "a_bl",
+filegroup(
+    name = "a",
     srcs = ["a.bzl"],
 )
 
 bzl_library(
-    name = "b_bl",
+    name = "b",
     srcs = ["b.bzl"],
-    deps = [":a_bl"],
 )
 
-filegroup(
-    name = "a_fg",
-    srcs = ["a.bzl"],
+bzl_library(
+    name = "c",
+    srcs = ["c.bzl"],
+    deps = [
+        ":a",
+        ":b",
+    ],
 )
 
-bzl_library(
-    name = "b_fg",
-    srcs = ["b.bzl"],
-    deps = [":a_fg"],
+bzl_library_consumer(
+    name = "consumer",
+    target = ":c",
 )
 
 build_test(
     name = "test_bzl_library",
-    targets = [
-        ":b_bl",
-        ":b_fg",
-    ],
+    targets = [":consumer"],
 )

tests/bzl_library/b.bzl

@@ -1,5 +1,3 @@
-"""b.bzl, which loads from a.bzl"""
+"""b.bzl, havin' a grand time"""
 
-load(":a.bzl", "A")
-
-B = A + 70
+B = 70

tests/bzl_library/bzl_library_consumer.bzl

@@ -0,0 +1,20 @@
+"""A rule that consumes a bzl_library target and asserts its providers are as expected."""
+
+load("//:bzl_library.bzl", "StarlarkLibraryInfo")
+
+def _bzl_library_consumer_impl(ctx):
+    files = sorted([t.basename for t in ctx.attr.target.files.to_list()])
+    if files != ["a.bzl", "b.bzl", "c.bzl"]:
+        fail("unexpected filenames from DefaultInfo: ", files)
+    files = sorted([t.basename for t in ctx.attr.target[StarlarkLibraryInfo].transitive_srcs.to_list()])
+    if files != ["a.bzl", "b.bzl", "c.bzl"]:
+        fail("unexpected filenames from StarlarkLibraryInfo: ", files)
+
+bzl_library_consumer = rule(
+    implementation = _bzl_library_consumer_impl,
+    attrs = {
+        "target": attr.label(
+            providers = [[StarlarkLibraryInfo]],
+        ),
+    },
+)

tests/bzl_library/c.bzl

@@ -0,0 +1,6 @@
+"""c.bzl, standin' on the shoulder of giants"""
+
+load(":a.bzl", "A")
+load(":b.bzl", "B")
+
+C = A + B