Errors are needed when referring to local things outside of the project

This currently includes:

• Referring to local plugins from project.conf using relative paths which lead outside of the project directory
• Referring to files or directories which lead outside of the project directory in local source path: configuration

Also relevant, is usage of symlinks in a project directory which lead outside of the project, but are in any way required to build the project, this can happen for both of the above mentioned cases (using proper paths which refer to symlinks which instead attempt to crawl out of the project directory).

In any of these cases, BuildStream should abort early with an error message.