[BuildStream] Preserving artifacts out of workspaces

[William Salmon <will salmon codethink co uk>]

I have been looking at, preserving artifacts out of workspaces, 
https://gitlab.com/BuildStream/buildstream/issues/512.

Overview
========

At the end of the ticket it mentions artifacts appearing in arbitrary 
location with in the sandbox however this seems to be at odds with, my 
understanding of, the behavior of the sand box, as at build time most of 
the sandbox is read only.

This means that the only place that artifacts should be is in 
subdirectories of the parent of the source directory.

Having Spoken to the originator of the ticket and some of those who have 
already been discussing the ticket, it seems that their most likely use 
case would be to have artifacts in the parent of the source directory 
(or in folders contained with in it).

There for if I make the *assumption*that all artifacts will reside in 
the source directories parent (or folders from it).

Then it seems to me that, of the Possible fixes in the issue, the first 
is the most simple and useful/flexible.

However it also seems that while we would want to support this behavior 
it will only occasionally be useful and change how workspaces behave for 
those already using buildstream. It may be best to add a switch to allow 
a "build-root" or "source", workspace.

Proposal
========

Allow for workspaces to be created with the parent of the source folder 
mounted rather than the source folder mounted.

This would be used when the user ran `bst workspace open element.bst 
DestDir --source-parent` but not affect the current behavior of `bst 
workspace open element.bst DestDir`.

A key requirement would be that the artifacts are loaded back in from 
the workspace for subsequent runs so that incremental builds can use 
previously build artifacts within arbitrary locations with in the 
build-root.

Implementation
==============

This means some implementation changes to the staging and unstaging to 
and from the sand box of the existing workspace code, and keeping track 
of what type of workspace it is etc etc.

And some front end changes to allow the `--source-parent` option to be 
passed to the work space and staging functions.

Thanks
Will