Make workspaces element-wide instead of source-specific

At present, BuildStream supports source-specific workspaces. This patch makes workspaces element-wide. This will bring workspaces closer to what the build area looks like inside the sandbox when the build actually happens.

As part of this change, format of .bst/workspaces.yaml file will also change. Previously, each element used to have a dict mapping each source to its workspace directory. Now, each element will directly map to its workspace directory.

If users have existing workspaces open, this patch tries to cope with it but in some cases, it is not possible to reliably convert workspace config from old format to new format. When an element has workspace open for just one source, we assume that to be the workspace directory. But if there were more than one workspaces associated with an element, BuildStream will simply error out describing the issue.

Note that the actual contents of the workspace directory are not changed in any case. So, if an element had multiple sources associated with it but only had workspace open for source, the other sources will not be automatically staged in the workspace unless the user runs workspace reset or something equivalent.

Part of #209 (closed).