How to combine multiple sections of pyproject.toml with the current file tree abstraction

[abravalheri]

In a recent discussion, we have identified a potential challenge for v5 and the adoption of pyproject.toml as a file where multiple configurations are added to.

Currently we can afford multiple feature/extensions because each of them maps to a different file when relevant. So we can have .coveragerc or .isort.cfg with independent behaviours (pyscaffold.operations.skip_on_update, pyscaffold.operations.no_overwrite, pyscaffold.operations.delete, pyscaffold.operations.add_permissions, etc...)

When all the configurations converge to a single file, that would be complicated.

We can extend the existing tree abstraction to the pyproject.toml file. This way each section of the file would be an independent leaf. There are 2 ways of implementing this (at least):

1. Actually write the sections into a temporary directory and then combine everything as a single file -- that might be not the most elegant way of doing things, but it is actually very practical. This way we don't have to change the way pyscaffold.operations work.
2. Implement this logically, without touching the file system. This is more tricky and force us to review the implementation of pyscaffold.operations (although it looks more elegant in principle). We might want to check if there is any library that abstracts the file system for an in memory file system that we could use.