Expose `defs.bzl` form `resolved_interpreter_os_alias` as a target (#659)

Co-authored-by: Alex Eagle <alex.eagle@robinhood.com>

python/private/toolchains_repo.bzl

diff --git a/python/private/toolchains_repo.bzl b/python/private/toolchains_repo.bzl
index e171e43..cbf0216 100644
--- a/python/private/toolchains_repo.bzl
+++ b/python/private/toolchains_repo.bzl
@@ -87,6 +87,7 @@
     build_contents = """\
 # Generated by python/repositories.bzl
 package(default_visibility = ["//visibility:public"])
+exports_files(["defs.bzl"])
 alias(name = "files",           actual = "@{py_repository}_{host_platform}//:files")
 alias(name = "py3_runtime",     actual = "@{py_repository}_{host_platform}//:py3_runtime")
 alias(name = "python_runtimes", actual = "@{py_repository}_{host_platform}//:python_runtimes")