Avoid hanging artifact cache tests

See merge request !964

This eliminates unneeded copies of the project directory for junctions
with a single local source.

Optimization for local junctions

See merge request !290

This improves readability.

This adds the batch() context manager.

This allows the sandbox to output messages using the plugin id.

This allows batching of all sandbox commands aross prepare() and
assemble().

Execute batched commands in a single shell script.

Command batching

Closes #675

See merge request !915

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.

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.

Searching for the workspace file is preferred to searching for the
project file for the moment, because it avoids the circumstance of
the workspaces being inside a directory where a higher-level directory
may have project.conf (but may not be a buildstream project).

This will benefit from being revisited when support for multiple
projects owning a workspace is supported - if we're inside a project and
running commands in a workspace that refers to multiple projects, we
want the one we're inside, not whichever added it first.

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.