derivepassphrase bug single-toplevel-module

Bug details: Move sequin and ssh_agent_client modules into derivepassphrase package

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)

The current layout, using three top-level Python packages derivepassphrase, sequin and ssh_agent_client, is error-prone:

• The sequin module and the ssh_agent_client package already are tightly coupled to the derivepassphrase package, insofar as their scope and functionality is solely dictated by the derivepassphrase package. sequin in particular is very special purpose, and unlikely to be useful in contexts other than passphrase generation.
• For 0.1.0 and 0.1.1, the Python wheels forgot to include sequin and ssh_agent_client, which led to broken installations.
• Version info via the __version__ attribute needs to be replicated across all three top-level packages.
• At least ssh_agent_client is likely enough a module/package name that we may expect name collisions in the future.

Since the only argument I have for keeping the packages separate is decoupling and independent evolution, which I think is completely mitigated by the de facto coupling to the derivepassphrase package anyway, I wish to integrate the sequin module and the ssh_agent_client package into the derivepassphrase package in the long run (and perhaps consolidate both types submodules).