refactor resource commands

[Anastasia]

Resources commands - 'juju charm resources", "juju resources", "juju attach" - need to be moved from resources/cmd under a common infrastructure in cmd package.

They are also under-tested. We need to add unit tests to exercise them.

[Anastasia (anastasia-macmood)]

This is not a small piece of work.

I have broken it into several parts to facilitate and expedite reviews:

1. straightforward move of resource/cmd into cmd/juju/resource;
2. breakout from component registration;
3. resource listener should use accessor function internal similar to store;
4. sanity feature testing for full-stack connectivity.

Part 1 against develop: https://github.com/juju/juju/pull/7680

[Anastasia (anastasia-macmood)]

After some thought and consideration, part 3 is slightly different - resource lister is passed into constructor instead. PR against develop (2.3-alpha1): https://github.com/juju/juju/pull/7682

[Anastasia (anastasia-macmood)]

Part 2 of the fix against develop (2.3-alpha1): https://github.com/juju/juju/pull/7686

[Anastasia (anastasia-macmood)]

Final part, with the actual feature test, against develop (2.3-alpha1): https://github.com/juju/juju/pull/7691