restrict visibility on filegroup target only consumed in 1 place (#605)

python/pip_install/BUILD

diff --git a/python/pip_install/BUILD b/python/pip_install/BUILD
index 41cac99..7727b29 100644
--- a/python/pip_install/BUILD
+++ b/python/pip_install/BUILD
@@ -26,7 +26,7 @@
         "//python/pip_install/extract_wheels:py_srcs",
         "//python/pip_install/parse_requirements_to_bzl:py_srcs",
     ],
-    visibility = ["//visibility:public"],
+    visibility = ["//python/pip_install/private:__pkg__"],
 )
 
 exports_files(