Fix bst source-checkout not working with open workspaces

Closes #807

See merge request !997

i.e. with a given directory and filename, check parent directories until
either a directory with the filename is found, or you reach the root of
the filesystem.

This is a part of #222

WorkspaceProject contains all the information needed to refer back to a
project from its workspace (currently this is the project path and the
element used to create this workspace)

This is stored within a WorkspaceProjectCache object, which keeps
WorkspaceProjects around so they don't need to be loaded from disk
repeatedly.

Workspaces has been extended to contain the WorkspaceProjectCache, and
will use it when opening and closing workspaces.

Context has been extended to contain a WorkspaceProjectCache, as there
are times when we want to use it before a Workspaces can be initialised
(looking up a WorkspaceProject to find the directory that the project is
in)

This is a part of #222

This is a part of #222

This is a part of #222

This is a part of #222

This is a part of #222

This is a part of #222

This is a part of #222

cli: Interactively warn if the user is trying to close the workspace they're using to load the project

This involves changes in:
* _stream.py:
  * Add the helper Stream.workspace_is_required()
* userconfig.yaml:
  * Add a default value for prompt.really-workspace-close-project-inaccessible
* _context.py:
  * Load the prompt 'really-workspace-close-project-inaccessible' from
    user config.
* cli.py:
  * If buildstream is invoked interactively, prompt the user to confirm
    that they want to close the workspace they're using to load this
    project.

This is a part of #222

This is a part of #222

This is a part of #222