Refactored export utils' generate_import_code function

[danthedaniel]

What does this PR do?

Begins the long journey of refactoring and cleaning up the export_utils module. Here I've refactored the generate_import_code function so that it treats data as data by placing all import statements for different operators/preprocessors into a dict.

The import code generated should also be much cleaner, as it will transform previous imports of the form:

from module.name import OperatorA
from module.name import OperatorB

into:

from module.name import OperatorA, OperatorB

And duplicate imports are not possible either

Where should the reviewer start?

In the export_utils.py module at the generate_import_code function.

How should this PR be tested?

I would recommend comparing the import statements in exported pipelines between this codebase and the existing codebase to verify they are equivalent.

Any background context you want to provide?

No

What are the relevant issues?

None

Screenshots (if appropriate)

Questions:

• Do the docs need to be updated?

No

• Does this PR add new (Python) dependencies?

No

[coveralls]

Coverage increased (+0.1%) to 28.808% when pulling 9edff29 on teaearlgraycold:eu_imports_ref into 8417028 on rhiever:master.