derivepassphrase bug no-stdlib-module-names

Bug details: Rename types submodules to _types

Class	bug	This is clearly an actual problem we want fixed.
Present-in	0.1.0 0.1.1 0.1.2 0.1.3
Fixed-in	c4a57f311710768cb18df717a73fd48a8a3077fe (0.2.0)

It appears to be a very bad idea to name a submodule similar to a standard library module, as some tools, e.g. vim’s keyword lookup (K), execute the code and then may run into “circular import” problems because the types submodule shadows the types standard library module.

So, rename the types submodules to _types.