Added get_app_config() to look up app configs by label.

Refactored get_app() to rely on that method.

get_app() starts by calling _populate(), which goes through
INSTALLED_APPS and, for each app, imports the app module and attempts to
import the models module. At this point, no further imports are
necessary to return the models module for a  given app. Therefore, the
implementation of get_app() can be simplified and the safeguards for
race conditions can be removed.

Besides, the emptyOK parameter isn't used anywhere in Django. It was
introduced in d6c95e93 but not actually used nor documented, and it has
just been carried around since then. Since it's an obscure private API,
it's acceptable to stop supporting it without a deprecation path. This
branch aims at providing first-class support for applications without a
models module eventually.

For backwards-compatibility, get_app() still raises ImproperlyConfigured
when an app isn't found, even though LookupError is technically more
correct. I haven't gone as far as to preserve the exact error messages.
I've adjusted a few tests instead.
Added get_app_configs() to iterate on app_config instances.

Refactored get_apps() to rely on that method.

This commit is fully backwards-compatible.
Deprecated get_app_package, get_app_path and get_app_paths.
Simplified handling of available_apps slightly.

It feels more natural for self.available_apps to contain app names (like
INSTALLED_APPS) than app labels, and this is easy to implement now.
Simplified register_models.

Since it's never called with more than one model at a time the current
signature is needlessly complicated.