Extract go_merge into its own package

This change is needed to support building gvisor for Fuchsia, which uses
Chromium's GN build system; at the time of writing, Fuchsia's Go support
does not include explicit enumeration of files, assuming instead that Go
binaries are always built from all Go source files in a given package.
Rather than extending Fuchsia's Go support, it is easier simply to
extract a separate package here.

PiperOrigin-RevId: 227133402
Change-Id: I1c64fff286d9c014b4bd1183b76023b35b60c720

tools/go_generics/BUILD

@@ -13,14 +13,6 @@ go_binary(
     deps = ["//tools/go_generics/globals"],
 )
 
-go_binary(
-    name = "go_merge",
-    srcs = [
-        "merge.go",
-    ],
-    visibility = ["//visibility:public"],
-)
-
 genrule(
     name = "go_generics_tests",
     srcs = glob(["generics_tests/**"]) + [":go_generics"],

tools/go_generics/defs.bzl

@@ -48,7 +48,7 @@ go_template = rule(
         "opt_types": attr.string_list(),
         "consts": attr.string_list(),
         "opt_consts": attr.string_list(),
-        "_tool": attr.label(executable = True, cfg = "host", default = Label("//tools/go_generics:go_merge")),
+        "_tool": attr.label(executable = True, cfg = "host", default = Label("//tools/go_generics/go_merge")),
     },
     outputs = {
         "out": "%{name}_template.go",

tools/go_generics/go_merge/BUILD

@@ -0,0 +1,9 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_binary")
+
+package(licenses = ["notice"])  # Apache 2.0
+
+go_binary(
+    name = "go_merge",
+    srcs = ["main.go"],
+    visibility = ["//visibility:public"],
+)