Reimplement addbuilddir() in C inside getpath.c, so as to execute it
at interpreter startup before importing any non-builtin modules.
Should fix #9589.
diff --git a/.bzrignore b/.bzrignore
index d2ba64d..2a16bcd 100644
--- a/.bzrignore
+++ b/.bzrignore
@@ -11,6 +11,7 @@
 build
 Makefile.pre
 platform
+pybuilddir.txt
 pyconfig.h
 libpython*.a
 python.exe