Allow bzl_library to depend on non-bzl_library targets

Notably, `filegroup`. `bzl_library` doesn't actually read anything from the `StarlarkLibraryInfo` provider, and requiring all deps to be other `bzl_library` targets is really painful for anyone loading .bzls from `@bazel_tools` or `@platforms` because those core modules/repos don't want a dependency on Skylib just for access to `bzl_library`.

The medium-term plan will be to move `bzl_library` into `@bazel_tools`; but before then, this can serve as a stop-gap.

bzl_library.bzl

@@ -54,10 +54,7 @@ bzl_library = rule(
         ),
         "deps": attr.label_list(
             allow_files = [".bzl", ".scl"],
-            providers = [
-                [StarlarkLibraryInfo],
-            ],
-            doc = """List of other `bzl_library` targets that are required by the
+            doc = """List of other `bzl_library` or `filegroup` targets that are required by the
 Starlark files listed in `srcs`.""",
         ),
     },

tests/bzl_library/BUILD

@@ -0,0 +1,32 @@
+load("//:bzl_library.bzl", "bzl_library")
+load("//rules:build_test.bzl", "build_test")
+
+bzl_library(
+    name = "a_bl",
+    srcs = ["a.bzl"],
+)
+
+bzl_library(
+    name = "b_bl",
+    srcs = ["b.bzl"],
+    deps = [":a_bl"],
+)
+
+filegroup(
+    name = "a_fg",
+    srcs = ["a.bzl"],
+)
+
+bzl_library(
+    name = "b_fg",
+    srcs = ["b.bzl"],
+    deps = [":a_fg"],
+)
+
+build_test(
+    name = "test_bzl_library",
+    targets = [
+        ":b_bl",
+        ":b_fg",
+    ],
+)

tests/bzl_library/a.bzl

@@ -0,0 +1 @@
+A = 30

tests/bzl_library/b.bzl

@@ -0,0 +1,2 @@
+load(":a.bzl", "A")
+B = A + 70