Gazelle now handles imports from `@bazel_tools` (#273)

`@bazel_tools` is tricky since it is effectively a part of the standard
library that can not have a `bzl_library` attached to it. As a simple
fix for this, `bzl_library` can have a srcs dependency on it so that it
includes the transitive closure of all of its dependencies.

`@bazel_tools` imports are rewritten into the `srcs` attribute since
they are `exports_files`ed from the @bazel_tools.

Co-authored-by: c-parsons <cparsons@google.com>

gazelle/bzl/gazelle.go

@@ -161,11 +161,23 @@ func (*bzlLibraryLang) Resolve(c *config.Config, ix *resolve.RuleIndex, rc *repo
 		// the index only contains absolute labels, not relative
 		impLabel = impLabel.Abs(from.Repo, from.Pkg)
 
+		if impLabel.Repo == "bazel_tools" {
+			// The @bazel_tools repo is tricky because it is a part of the "shipped
+			// with bazel" core library for interacting with the outside world.
+			// This means that it can not depend on skylib. Fortunately there is a
+			// fairly simple workaround for this, which is that you can add those
+			// bzl files as `deps` entries.
+			deps = append(deps, imp)
+			continue
+		}
+
 		if impLabel.Repo != "" || !c.IndexLibraries {
 			// This is a dependency that is external to the current repo, or indexing
 			// is disabled so take a guess at what hte target name should be.
 			deps = append(deps, strings.TrimSuffix(imp, fileType))
-		} else {
+			continue
+		}
+
 		res := resolve.ImportSpec{
 			Lang: languageName,
 			Imp:  impLabel.String(),
@@ -182,7 +194,6 @@ func (*bzlLibraryLang) Resolve(c *config.Config, ix *resolve.RuleIndex, rc *repo
 			deps = append(deps, depLabel.String())
 		}
 	}
-	}
 
 	sort.Strings(deps)
 	if len(deps) > 0 {

gazelle/bzl/testdata/bazel_tools/BUILD.in

@@ -0,0 +1,6 @@
+# Some comment to be preserved
+
+filegroup(
+    name = "allfiles",
+    srcs = glob(["**"]),
+)

gazelle/bzl/testdata/bazel_tools/BUILD.out

@@ -0,0 +1,15 @@
+load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
+
+# Some comment to be preserved
+
+filegroup(
+    name = "allfiles",
+    srcs = glob(["**"]),
+)
+
+bzl_library(
+    name = "foo",
+    srcs = ["foo.bzl"],
+    visibility = ["//visibility:public"],
+    deps = ["@bazel_tools//tools/build_defs/repo:http.bzl"],
+)

gazelle/bzl/testdata/bazel_tools/foo.bzl

@@ -0,0 +1,10 @@
+"""
+Doc string
+"""
+
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+def wrapped_http_archive(**kwargs):
+    http_archive(
+        **kwargs
+    )