Allow spanner-cli to run under an environment for proper package dependency injection

[techgerm]

Problem:

• Migrations don't currently work using the spanner-cli if your project is a package and you've imported other helpful modules into some of your migrations files.
• Reference conversation for more details here: Migrations - Add support for package imports #12

Proposed Solution:

• We could get the spanner-cli tool to use a env.py file similar to how alembic does it that then it could use to properly inject package structure/dependencies.