[Python] Refine higher level dataset API

[asfimport]

Provide a more intuitive way to construct nested dataset:

1. instead of using confusing factory function
   dataset([
        factory("s3://old-taxi-data", format="parquet"),
        factory("local/path/to/new/data", format="csv")
   ])
   
1. let the user to construct a new dataset directly from dataset objects
   dataset([ 
       dataset("s3://old-taxi-data", format="parquet"),
       dataset("local/path/to/new/data", format="csv")
   ])

In the future we might want to introduce a new Dataset class which wraps functionality of both the dataset actory and the materialized dataset enabling optimizations over rediscovery of already materialized datasets.

Reporter: Krisztian Szucs / @kszucs
Assignee: Krisztian Szucs / @kszucs

PRs and other links:

• GitHub Pull Request #6505

_{Note: This issue was originally created as ARROW-7965. Please see the migration documentation for further details.}

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
This depends on ARROW-8164, but if that gets merged quickly, it would be nice tackle this issue for 0.17 since that would enable us to remove factory() from the high-level user API (which wasn't there yet in 0.16, so this would avoid it ever being in a released version)

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 6505
#6505