go_repository: don't pass default values on the command line (#1294)

go_repository will no longer pass arguments to Gazelle on the command
line when the corresponding attributes are not explicitly set.

This gives Gazelle more control over the defaults. For example,
Gazelle allows an inferred proto mode to override the default mode,
but not when the default mode is explicitly requested.

Related bazel-contrib/bazel-gazelle#104

go/private/go_repository.bzl

@@ -87,15 +87,18 @@ def _go_repository_impl(ctx):
     # Build file generation is needed
     _gazelle = "@io_bazel_rules_go_repository_tools//:bin/gazelle{}".format(executable_extension(ctx))
     gazelle = ctx.path(Label(_gazelle))
-    cmds = [gazelle, '--go_prefix', ctx.attr.importpath, '--mode', 'fix',
-            '--repo_root', ctx.path(''),
-            "--build_tags", ",".join(ctx.attr.build_tags),
-            "--external", ctx.attr.build_external,
-            "--proto", ctx.attr.build_file_proto_mode]
+    cmd = [gazelle, '--go_prefix', ctx.attr.importpath, '--mode', 'fix',
+            '--repo_root', ctx.path('')]
     if ctx.attr.build_file_name:
-        cmds.extend(["--build_file_name", ctx.attr.build_file_name])
-    cmds.append(ctx.path(''))
-    result = env_execute(ctx, cmds)
+      cmd.extend(["--build_file_name", ctx.attr.build_file_name])
+    if ctx.attr.build_tags:
+      cmd.extend(["--build_tags", ",".join(ctx.attr.build_tags)])
+    if ctx.attr.build_external:
+      cmd.extend(["--external", ctx.attr.build_external])
+    if ctx.attr.build_file_proto_mode:
+      cmd.extend(["--proto", ctx.attr.build_file_proto_mode])
+    cmd.append(ctx.path(''))
+    result = env_execute(ctx, cmd)
     if result.return_code:
       fail("failed to generate BUILD files for %s: %s" % (
           ctx.attr.importpath, result.stderr))
@@ -129,8 +132,8 @@ go_repository = repository_rule(
 
         # Attributes for a repository that needs automatic build file generation
         "build_external": attr.string(
-            default = "external",
             values = [
+                "",
                 "external",
                 "vendored",
             ],
@@ -146,8 +149,8 @@ go_repository = repository_rule(
         ),
         "build_tags": attr.string_list(),
         "build_file_proto_mode": attr.string(
-            default = "default",
             values = [
+                "",
                 "default",
                 "disable",
                 "legacy",