transaction, moduler: Do not export PAM conv handler function to modules

This function is only needed when using go PAM for creating applications
so it's not something we expect to have exported to library modules.

To prevent this use an `asPamModule` tag to prevent compilation of
application-only features.

example-module/pam_module.go

@@ -1,6 +1,6 @@
 // Code generated by "pam-moduler "; DO NOT EDIT.
 
-//go:generate go build "-ldflags=-extldflags -Wl,-soname,pam_go.so" -buildmode=c-shared -o pam_go.so
+//go:generate go build "-ldflags=-extldflags -Wl,-soname,pam_go.so" -buildmode=c-shared -o pam_go.so -tags go_pam_module
 
 // Package main is the package for the PAM module library.
 package main