Disentangle import cycles

[tseaver]

Some local non-testing imports are explicitly marked as related to cycles:

$ git grep -n circular
gcloud/datastore/dataset.py:125:        # This import is here to avoid circular 
gcloud/datastore/entity.py:157:        # This is here to avoid circular imports.

Others are not so marked:

gcloud/datastore/__init__.py:64:    from gcloud import credentials
gcloud/datastore/__init__.py:65:    from gcloud.datastore.connection import Connection
gcloud/datastore/dataset.py:73:        from gcloud.datastore.query import Query
gcloud/datastore/dataset.py:86:        from gcloud.datastore.entity import Entity
gcloud/datastore/dataset.py:99:        from gcloud.datastore.transaction import Transaction

gcloud/storage/__init__.py:65:    from gcloud import credentials
gcloud/storage/__init__.py:66:    from gcloud.storage.connection import Connection
gcloud/storage/iterator.py:158:        from gcloud.storage.bucket import Bucket
gcloud/storage/iterator.py:186:        from gcloud.storage.key import Key

We should refactor to fix the cycles, allowing us to normalize those imports.

[dhermes]

I like it!

[tseaver]

To prevent regressions, (re)enable pylint's 'cyclic-imports' check (disabled in #238) when we're done.

[dhermes]

@tseaver are you working on this?

If not, I'm gonna assign myself. I suppose that should be part of our workflow anyhow, "self-assigning" when working on something.

[tseaver]

Go ahead.

I think assignment (either self- or from @silvolu) is a reasonable "gateway" to avoiding duplication.

[dhermes]

PR #267 begins this

[tseaver]

After merging #268 and #269, the last step is to drop the exclusion of 'cycle-imports' from the two pylintrc files.