Skip the default heroku-postgresql addon if DATABASE_URL is set

Currently the Python buildpack's `bin/release` step sets a default `heroku-postgresql` addon if the app has a `manage.py` script.

However for use-cases where the app wishes to use a different Heroku addon, or a non-Heroku DB, this default addon at best is unused and at worst causes confusion.

To resolve this the buildpack could skip the default addon if `DATABASE_URL` were already set at the time of the first build.

This would cause the default addon to be skipped if either:
1. the app had another addon attached prior to the first build (that provided the `DATABASE_URL` env var)
2. `DATABASE_URL` was manually set (eg via review app apps.json `env` list) then the default DB would be skipped

For more background see the discussion in:
https://github.com/heroku/heroku-buildpack-python/pull/1056

Note: The buildpack API does not provide the env dir location during release. As such the env var would need to be checked during `bin/compile` and either stored until `bin/release`, or else the release step refactored such that the payload is generated during `bin/compile` and saved to disk, and then `bin/release` simply returns the already-generated payload. The latter is what the Ruby buildpack does.