Merge pyproject.toml files for starters in line with python src-layout

[DimedS]

Description

Continuation from #2280.

The template project and starters currently contain two pyproject.toml files, one in the root directory and another in the src folder. This arrangement is not aligned with the python src-layout package structure. The two files should be merged into a single file in the root folder. This breaking change should be associated with the Kedro 0.19 release because it will affect kedro package command.

[astrojuanlu]

#2926 did this for the Kedro default template, now this needs replication on the starters 💪🏽

[SajidAlamQB]

This issue is blocking the PySpark add-ons work, #3169, specifically with the spaceflights-pyspark starter which needs to align with the core template structure in develop for it to work properly. I spoke to @DimedS and he said this needs to be merged when we release 0.19.0 but is there a way to get this out earlier?

Maybe we just do it for spaceflights-pyspark for now? kedro-org/kedro-starters#164

What do others think? @DimedS @astrojuanlu @merelcht

Update: Based on @merelcht comments in stand-up this should be fine to go in. 👍

[DimedS]

What do others think? @DimedS @astrojuanlu @merelcht

In my opinion that ticket is continuation of #2926 that Juan Lu merged into develop because it's a breaking change need to be in 0.19 release. But looks like that we need to update starters now (before kedro 0.19) to be able to continue development smoothly.