Having bidirectional channels is not a recommended practice, as it could
introduce deadlocks.  It is a very niche use case, so it is better to
remove it.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

`Peekable` was a class that allowed users to peek at the latest value in
a channel, without consuming anything. Even when useful in principle, it
is a special case that breaks the whole channel abstraction, and can
easily be emulated by just using a normal receiver and caching the last
value.

Also the following symbols are removed as they are not longer used:

* 'Broadcast.new_peekable`
* `Receiver.into_peekable`
* `ReceiverInvalidatedError`

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

There is little gain in importing type variables from other modules, as
it adds unncecessary dependencies between modules. Type variables are
just an artifact to define stuff locally.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Also move the receiver exceptions from `_exceptions` to `_receiver` and
the sender exceptions from `_exceptions` to `_sender`.

This avoids circular imports.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

We don't really want end users to have access to the concrete types, and
their custom methods. We want them to use the base types, so we can
change the implementation without breaking their code and ensuring
a common interface for all senders and receivers.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

We don't want to expose the implementation of the sender and receiver at
all, so we make them private to the module.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Now that the sender and receiver implementations are private, there is
no need to use aliases for the base classes.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

These are core component to work with channels, so they are moved to the
`frequenz.channels` top level module.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

We move the `Event`, `FileWatcher` and `Timer` receivers to their own
separate public modules. Since they are not core components and users
might only need to use some of them, it is more clear to have them
separated.

Also in the case of the `FileWatcher`, it requires an extra dependency,
which we could potentially have as an optional dependency, so users that
don't need it don't need to install it.

In the future we might distribute these also as separate python packages
(wheels).

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Removing nested classes avoids having to use hacky constructions, like
requiring the use of `from __future__ import annotations`, types as
strings and confusing the `mkdocstrings` tools when extracting and
cross-linking docs.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

This is from the beta-2 release, so we clear it out until we are ready
for the new release.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>