Fix use of //go with Bzlmod by removing dependency on `@org_golang_…

…x_sys` (#3512)

* Fix use of `//go` by exposing `@org_golang_x_sys` when using Bzlmod

Running `bazel run //go` (or `bazel run @rules_go//go` from another module) fails when using Bzlmod because the `@org_golang_x_sys` repository is not visible. Without Bzlmod, the repository is loaded through a WORKSPACE dependency and the error does not happen.

Two-step minimal reproduction from root of this repo:

1. `touch WORKSPACE.bzlmod`
2. `bazel run --enable_bzlmod //go`

Here's the error that produces:

```shell
$ bazel run --enable_bzlmod //go
DEBUG: [email protected]/MODULE.bazel:7:6: WARNING: The bazel_gazelle Bazel module is still highly experimental and subject to change at any time. Only use it to try out bzlmod for now.
WARNING: For repository 'com_google_protobuf', the root module requires module version [email protected], but got [email protected] in the resolved dependency graph.
ERROR: /Users/brice/Documents/Code/rules_go/go/tools/go_bin_runner/BUILD.bazel:11:11: no such package '@[unknown repo 'org_golang_x_sys' requested from @]//unix': The repository '@[unknown repo 'org_golang_x_sys' requested from @]' could not be resolved: No repository visible as '@org_golang_x_sys' from main repository and referenced by '//go/tools/go_bin_runner:go_bin_runner_lib'
ERROR: Analysis of target '//go:go' failed; build aborted: 
INFO: Elapsed time: 3.043s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (45 packages loaded, 865 targets configured)
    currently loading: go/platform
ERROR: Build failed. Not running target
```

Here's the point in the BUILD file where this repository is being referenced:

https://github.com/bazelbuild/rules_go/blob/e558f56df9995f571e49de336c3831d407d9ff70/go/tools/go_bin_runner/BUILD.bazel#L25

* Remove `process_unix.go` and use `process.go` for all platforms. Eliminates dependency on `@org_golang_x_sys`.

* Add Bzlmod test for //go

---------

Co-authored-by: Fabian Meumertzheim <[email protected]>

go/tools/go_bin_runner/BUILD.bazel

@@ -12,18 +12,13 @@ go_library(
     name = "go_bin_runner_lib",
     srcs = [
         "main.go",
-    ] + select({
-        "@platforms//os:windows": ["process.go"],
-        "//conditions:default": ["process_unix.go"],
-    }),
+        "process.go",
+    ],
     importpath = "github.com/bazelbuild/rules_go/go/tools/go_bin_runner",
     visibility = ["//visibility:private"],
     deps = [
         "//go/runfiles",
-    ] + select({
-        "@platforms//os:windows": [],
-        "//conditions:default": ["@org_golang_x_sys//unix"],
-    }),
+    ],
 )
 
 go_binary(

go/tools/go_bin_runner/process.go

@@ -1,5 +1,3 @@
-//go:build !unix
-
 package main
 
 import (

tests/bcr/BUILD.bazel

@@ -45,3 +45,11 @@ go_test(
     embed = [":mockable"],
     deps = ["@my_rules_go//extras/gomock"],
 )
+
+sh_test(
+    name = "go_version_test",
+    srcs = ["go_version_test.sh"],
+    data = ["@my_rules_go//go"],
+    env = {"GO_TOOL_RLOCATION": "$(rlocationpath @my_rules_go//go)"},
+    deps = ["@bazel_tools//tools/bash/runfiles"],
+)

tests/bcr/go_version_test.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+
+# --- begin runfiles.bash initialization v3 ---
+# Copy-pasted from the Bazel Bash runfiles library v3.
+set -uo pipefail; set +e; f=bazel_tools/tools/bash/runfiles/runfiles.bash
+source "${RUNFILES_DIR:-/dev/null}/$f" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "${RUNFILES_MANIFEST_FILE:-/dev/null}" | cut -f2- -d' ')" 2>/dev/null || \
+  source "$0.runfiles/$f" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "$0.runfiles_manifest" | cut -f2- -d' ')" 2>/dev/null || \
+  source "$(grep -sm1 "^$f " "$0.exe.runfiles_manifest" | cut -f2- -d' ')" 2>/dev/null || \
+  { echo>&2 "ERROR: cannot find $f"; exit 1; }; f=; set -e
+# --- end runfiles.bash initialization v3 ---
+
+GO_TOOL=$(rlocation "$GO_TOOL_RLOCATION")
+# Set runfiles variables for subprocess.
+runfiles_export_envvars
+# Simulate a bazel run environment.
+export BUILD_WORKING_DIRECTORY=$(pwd)
+[[ "$("$GO_TOOL" version)" =~ ^go ]]