Allow running bst commands from workspace directory

Say I want to open a workspace for some GNOME module, let's use gnome-desktop. I do this:

$ cd ~/Projects/gnome-build-meta
$ bst workspace open core/gnome-desktop.bst ~/Projects/gnome-desktop

Now I make some changes to the module and want to build it:

$ cd ~/Projects/gnome-desktop
# change stuff
$ bst build core/gnome-desktop.bst

It doesn't work, because I need to be in my gnome-build-meta directory for any bst commands to work. This means I either need to keep two different terminal windows open at all times, or else continually cd between the workspace directory and the gnome-build-meta directory whenever I want to switch between running bst commands or git commands.

The workspace ought to somehow know that it was checked out from gnome-build-meta, to avoid this problem. It should be doable using git config; bst could have done something like git config set bst.projects-dir ~/Projects/gnome-build-meta after checking out the git repo, so it would know where to look for the projects and running bst commands would work anywhere within the gnome-desktop repo. That's just one possibility; no doubt there are better ways to make it work.

Update: a proposal has been sent to the mailing list that covers this ticket.

Update by Jonathan Maw on 19th November:

Following a summary E-mail, there are multiple mostly-discrete steps to this:

1. Create a .bstproject.yaml file when a workspace is opened, and remove it when the workspace is closed.
2. When invoking buildstream in a workspace that exists outside a project, read .bstproject.yaml to find the path to the project, and run all commands that way.
3. As 2, but the element name is optional, defaulting to the element that this source belongs to.
4. When workspaces are opened with --force, keep track of multiple projects accessing this workspace. bst commands run inside the workspace default to the first project to open this workspace.