build(python): simplify rpycdec vendor layout

third_party/python/BUILD.bazel

@@ -1,10 +1,11 @@
-load("@rules_python//python:defs.bzl", "py_binary")
+load("@rules_python//python:defs.bzl", "py_binary", "py_library")
 
 package(default_visibility = ["//visibility:public"])
 
-alias(
+py_library(
     name = "rpycdec",
-    actual = "//third_party/python/rpycdec-0.1.13+489e9c6:rpycdec",
+    srcs = glob(["rpycdec/src/**/*.py"]),
+    imports = ["rpycdec/src"],
 )
 
 py_binary(