Add sources to bst shell

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/413)
In GitLab by [[Gitlab user @knownexus]](https://gitlab.com/knownexus) on May 30, 2018, 13:59

## Background

[//]: #
We want to be able to run software debuggers & co when we run `bst shell`

## Task description

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

* [ ] Modify `bst shell` to include the sources of all open workspaces in a given project
* [ ] Potentially add a flag to enable/disable the feature (undecided)

## Steps

[//]: # (Acceptance criteria should follow the S.M.A.R.T. principle https://en.wikipedia.org/wiki/SMART_criteria )

* [ ] Find the point were artifacts are added to the sandbox
* [ ] Get a list of all open workspaces
* [ ] Get the build-root for the elements of each workspace
* [ ] Recreate the full path of each workspace inside the sandbox
* This is to allow existing debug logs and paths to find the right path
* In the case of conflicting paths between workspaces, the shell will abort and instruct the user to close some/all of the conflicting workspaces
* [ ] Copy the contents of the workspaces into their corresponding directory
* [ ] Add tests to confirm that the files were copied into the sandbox

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Inizia dall’implementazione di `bst shell` e dal punto in cui gli artefatti vengono aggiunti alla sandbox. Traccia il modo in cui i workspace aperti e i relativi elementi espongono le radici di build, quindi determina come ricreare i percorsi dei workspace e come gestire i conflitti. Il lavoro è completato quando i contenuti dei workspace compaiono nei percorsi previsti della sandbox, i conflitti interrompono l’operazione fornendo indicazioni e i test confermano i file copiati.

Scritto dal modello di indicizzazione a partire dal testo della issue.