Support multiple workspaces in one repository

[lukaszwawrzyk]

This issue is about a corner case when someone has, say, metamonorepo, i.e. one git repository contains multiple Bazel projects. Each project would be treated separately, with separate config.

Users should be able to configure relative paths within repositories where workspaces are located. By default it would be single element list with root directory ["."].

There are multiple ways to approach this.

• We can combine it with approach from #39, i.e. each "subworkspace" would be specified as a workspace, either as a path to the workspace in the local file system or git url + relative path using some syntax.
• We can auto detect bazel workspaces in given repository and only run for those who have the bazel-steward config file.