Gazelle now handles imports from @bazel_tools

`@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.

gazelle/bzl/gazelle.go

@@ -150,33 +150,50 @@ func (*bzlLibraryLang) Resolve(c *config.Config, ix *resolve.RuleIndex, rc *repo
 		return
 	}
 
+	extraSrcs := make([]string, 0, len(imports))
 	deps := make([]string, 0, len(imports))
 	for _, imp := range imports {
+		if strings.HasPrefix(imp, "@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 `srcs` entries.
+			extraSrcs = append(extraSrcs, imp)
+			continue
+		}
+
 		if strings.HasPrefix(imp, "@") || !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 {
-			res := resolve.ImportSpec{
-				Lang: languageName,
-				Imp:  imp,
-			}
-			matches := ix.FindRulesByImport(res, languageName)
+			continue
+		}
 
-			if len(matches) == 0 {
-				log.Printf("%s: %q was not found in dependency index. Skipping. This may result in an incomplete deps section and require manual BUILD file intervention.\n", from.String(), imp)
-			}
+		res := resolve.ImportSpec{
+			Lang: languageName,
+			Imp:  imp,
+		}
+		matches := ix.FindRulesByImport(res, languageName)
 
-			for _, m := range matches {
-				deps = append(deps, m.Label.String())
-			}
+		if len(matches) == 0 {
+			log.Printf("%s: %q was not found in dependency index. Skipping. This may result in an incomplete deps section and require manual BUILD file intervention.\n", from.String(), imp)
+		}
+
+		for _, m := range matches {
+			deps = append(deps, m.Label.String())
 		}
 	}
 
 	sort.Strings(deps)
 	if len(deps) > 0 {
 		r.SetAttr("deps", deps)
 	}
+
+	sort.Strings(extraSrcs)
+	if len(extraSrcs) > 0 {
+		r.SetAttr("srcs", append(r.AttrStrings("srcs"), extraSrcs...))
+	}
 }
 
 var kinds = map[string]rule.KindInfo{

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 +1,17 @@
+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",
+        "@bazel_tools//tools/build_defs/repo:http.bzl",
+    ],
+    visibility = ["//visibility:public"],
+)

gazelle/bzl/testdata/bazel_tools/foo.bzl

@@ -0,0 +1,6 @@
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+def wrapped_http_archive(**kwargs):
+    http_archive(
+        **kwargs
+    )