modules: Change name only when _GO_PLUGINS_DIR set

The test that first caught this was `modules/main: --imported`. Added
the new `modules/use: import builtin module` test case to cover any
regression in a more obvious way.

Author: mbland

lib/internal/use

@@ -147,7 +147,8 @@ for __go_module_name in "$@"; do
   # namespace itself is flat, this might lead to hard-to-debug collisions if
   # functions and variables get redefined. Keeping a flat module name namespace
   # allows us to detect such potential collisions and issue a warning below.
-  if [[ "$__go_module_file" =~ ^$_GO_PLUGINS_DIR ]]; then
+  if [[ -n "$_GO_PLUGINS_DIR" &&
+        "$__go_module_file" =~ ^$_GO_PLUGINS_DIR ]]; then
     __go_module_name="${__go_module_file##*/plugins/}"
     __go_module_name="${__go_module_name/\/bin\///}"
     __go_module_name="${__go_module_name/\/lib\///}"

tests/modules/use.bats

@@ -83,6 +83,13 @@ teardown() {
   assert_success ''
 }
 
+@test "$SUITE: import builtin module" {
+  run "$TEST_GO_SCRIPT" 'complete'
+  assert_success 'module: complete' \
+    "source: $_GO_CORE_DIR/lib/complete" \
+    "caller: $TEST_GO_SCRIPT:3 main"
+}
+
 @test "$SUITE: error if nonexistent module specified" {
   run "$TEST_GO_SCRIPT" 'bogus-test-module'