Removes the brightway2 imports and replaces them with the imports for their individual packages

Implements changes for #14

Checklist

[ ] Keep pull requests small so they can be easily reviewed.
[ ] Update the documentation, please follow the numpy style guide.
[ ] Update tests.
[ ] Categorize the PR by setting a good title and adding one of the labels: bug, feature, ui, change, documentation, breaking, ci as they show up in the changelog.
[ ] Link this PR to related issues by using closing keywords.
[ ] Add a milestone to the PR for the intended release.
[ ] Request a review from another developer.