### Check existing issues - [x] I've verified that this request isn't described by any existing issues. ### Request Linters need access to the effective base to provide base-specific handling. Currently, they only have access to the `base` key and not the `build-base`, since they can have access to the SnapMetadata class. This means that the linters don't know the effective base of a bare base snap. ### The problem it solves So the GPU linter can provide the correct help link.