[BuildStream] Optional virtual directory support [Was: BuildBox transitioning]

[Tristan Van Berkom <tristan vanberkom codethink co uk>]

Hi all,

Forking this thread since I noticed this, I think it's a good thing to
raise before I forget about it...

On Mon, 2019-05-20 at 21:06 +0900, Tristan Van Berkom via buildstream-
list wrote:
> Hi,
[...]
> [...]
> > If this plan is acceptable I imagine the first MR would cover:
> > ==============================================================
> >
> > * The first implementation of the new sandbox.
> >
> > * Report nice errors rather than bugs for those trying out the new
> > sandbox, if they use parts that are not finished, for example: 
> >   - warning due to limited bst shell support 
> >   - error for element plugins that don't support the virtual
> > directory API 

Interestingly, it seems that elements which don't support virtual
directory API were never discussed since we decided to break API and
create BuildStream 2.

Frankly, the only reason why virtual directory support was ever
optional, was as a measure to ensure API compatibility in what was a
stable API.

Now that we are working on creating new API, is there any reason at all
that we would allow this to be optional for plugins to implement ?

Cheers,
    -Tristan