WorkspaceSource.init_workspace raises an exception so it is no longer
necessary to retain the original source objects of the loaded element.

Also rename it to 'node_subst_sequence_vars' to mimic 'node_subst_vars'.

This is now handled in Context.prepare_fork().

This aligns the method name with has_open_grpc_channels().

Remove XFAIL mark from test_workspace_visible and remove the explicit
SourceCache.commit() in the workspace source plugin. Allow buildstream
to handle the commit logic.

Add handling for non-cached workspace sources in
`source.Source._generate_keys()`.

Workspaces will be handled via the workspace source plugin methods. This
does not currently support reflecting build artifacts in the open
workspace.

tests: Mark incremental workspace builds as xfail (strict)

The `workspace.init_workspace()` call should wrap
`source._init_workspace` for held sources to support those sources not
publishing `BST_VIRTUAL_DIRECTORY`

This object owns a directory digest attribute used inplace of the source
ref.

`track` and `fetch` become noop methods and the workspace is imported
into the CAS in the call to `get_unique_key` which also sets the digest
attribute and owns that Directory object. The directory is referenced
during stage to import directly to the virtual directory object.

Importing is expected to be expensive and will be
optimised in future.

When the unique key is retrieved for the workspace source it will also
be commited to the sourcecache. The logic for this source is still a
slight variant on other sources since it cannot itself be expected to
be in the cache when it's opened.

In the source preflight method the preflights of the held sources must
be called.