Custom filter modules not in site-packages

[moggers87]

Filter modules defined in the current working directory (or anywhere that's not site packages) are not easily used (you get an import error) and it is impossible to use import_module to do relative imports without specifying a package. A quick workaround for this is to prepend PYTHONPATH=. to every exhibition command.

We should either:

• detect when a relative import is being used set the cwd as the package, or
• allow the user to set additional import search paths via site.yaml