Move some imports around to depend less on having an installed system

It's nice, for unit testing purposes, if some basic modules can be
imported even if not running in an installed system, despite them not
really including that much functionality that is usable in that setting.

This commit makes it so, and adds unit tests that simply check that the
modules can be imported.
Extend operation parameter handling

Extend operation parameter handling to support higher-level parameter
type specifications such as Review, Repository and Commit, which means
the input value can be some basic type (typically int or str) that we can
unambiguously translate into the requested object, and that the value
actually passed to the operation's implementation is a high-level object
(such as dbutils.Review, gitutils.Repository or gitutils.Commit.)

Also split operation/__init__.py into a few different modules, reducing
the actual __init__.py into just importing selected names from the new
modules.  It had grown slightly out of control, in particular with the
other changes made here.
Use operation.Review for review argument to /{prepare,revert}rebase

These operation are used extensively in tests, so are "safe" to modify.
Use operation.Repository for repository argument to /submitreview

The operation is used by four tests.  Adjust three of them to supply
the argument in different ways, so that we end up with these variants:

  { "repository": 1, ... }
  { "repository": "critic", ... }
  { "repository_id": 1, ... }
  { "repository_name": "critic", ... }
Use operation.User for subject argument to some operations

The operations are /setgitemails, /changepassword and /addemailaddress,
all of which we have tests for.

The argument to /setgitemails was renamed from "user_id" to "subject" (or
"subject_id"/"subject_name".)

Also delete the VerifyEmailAddress class, which was never finished and not
hooked up.  (The use-case was that an administrator would be able to use
it to force-verify another user's unverified email address, but this never
happened.)