`bst shell` could use splitting into artifact/source commands

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1152)
In GitLab by [[Gitlab user @tlater]](https://gitlab.com/tlater) on Oct 2, 2019, 17:11

## Background

[//]: # (Provide a background or the root/source that justifies this task or action.)

While writing #1151 I realized that `bst shell` would make more sense if it was split into `bst artifact shell` and `bst source shell`, where `bst source shell` would take on the jobs of the former `bst shell -b`.

I think this is a worthwhile thought, because `bst shell -b/--build` has [confused others in the past](), and makes the concept of build trees more complex, since it only applies to one kind of shell.

## Task description

[//]: # (Short summary of the action to be executed)

`bst shell -b` should be renamed to `bst source shell`, and `bst shell` should be renamed to `bst artifact shell`.

This probably warrants some more discussion, since it's deprecating another major command.

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[//]: # (To review information about possible relevant labels for this issue please view the list of labels: https://gitlab.com/BuildStream/buildstream/labels)

Start by reviewing the linked GitLab issue and the existing `bst shell` command entry point. Trace how `-b/--build` currently changes shell behavior and identify the deprecation implications. Done means the command split is agreed, with `bst artifact shell` replacing `bst shell` and `bst source shell` replacing `bst shell -b`.

Written by the indexing model from the issue text.