[Feature] Include files from upstream directories for Jinja2 Templates

[roelschr]

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This functionality existed (see https://github.com/databrickslabs/dbx/issues/191) but has been removed at some point with refactors. @renardeinside any special reason or would you accept new PRs to re-add this feature to dbx? It seems it's just a matter of adding curdir's absolute path to the FileSystemLoader here.