In order for other projects to make use of our APIs, they must not
reside underneath the internal directory. This moves the protobuf
definitions to a top-level "api" directory and scopes them according to
their domain. This change also removes generated code from the gitignore
file so that users don't have to generate the code themseleves.
Signed-off-by: Andrew Rynhard <andrew@andrewrynhard.com>
