* Add test illustrating import issue

* move layer materialization for shuffle

* add missing layers.py file and revise org a bit

* move layers.py

* remove xfail

* use import rather than pickle for functions

* fix importlib error and use dict

* roll back test changes from 7374 (let failures be resolved in that PR)

* remove debug print

* move Shuffle layers to layers.py completely

* moving moving BroadcastJoinLayer to layers.py

* tweak CallableLazyImport

* move BlockwiseCreateArray into layers.py

* import test coverage

* incorperate testing idea from 7374

* comment tweaks

* introduce new test_layers.py module

* update comment in test

* Update dask/layers.py

Co-authored-by: James Bourbeau <jrbourbeau@users.noreply.github.com>

* Only use CallableLazyImport when the graph is materialized within __dask_distributed_unpack__

* remove obsolete annotation handling

* Update dask/layers.py

Co-authored-by: James Bourbeau <jrbourbeau@users.noreply.github.com>

* _construct_graph fix

* migrate csv code

* migrate orc changes

* basic parquet migration - still need to handle serialization

* add require_pickle option to DataFrameIOLayer

* update testing

* change 'column culling' language to 'column projection'

* use ConcatAxesWrapper to avoid inline task

* add test coverage

* use serialize instead of pickle

* use simpler SerializedFunction wrapper

* update to rely on distributed#4575

* serialization experiments

* align with current status of distributed-4641

* update/fix comment

* align with latest distributed#4641 state

* re-align with James' suggestion

* Temporarily point to Distributed PR #4641

* point CI back to distributed main branch

* avoid using distributed 2021.03.0 for mindeps test

* start aligning with #7455

* begin requiring io_deps elements to inherit from base class

* more simplification

* fix indices init

* Add .git suffix

* Add explicit pip

* serialization cleanup

* address dep-name issue

* strip out unnecessary import logic

* use output_blocks in pack

* more code review suggestions

* use clearer tmp variable name

* avoid io_deps copy

* dictionary comp for BlockwiseDepDict pack

* start working on csv problem

* handle csv nested-task problem

* further cleanup

* apply fix

* add produces_tasks

* use place-holder required_indices variable

* CreateArrayDeps fix

* remove extra name def

* Change test ordering

* fix dumps_function mistake and account for required_indices being an empty collection

* roll back (breaking) large_graph_objects change

* minor blockwise changes to address code-review

* more cleanup

* use  for column projection in csv

* updating some comments

* improve documentation

* add project_columns to the functions to make things a bit more explicit for now

* move timeseries to Blockwise

* remove commented code

* add daily-stock

Co-authored-by: James Bourbeau <jrbourbeau@gmail.com>
Co-authored-by: James Bourbeau <jrbourbeau@users.noreply.github.com>