[all] OpenTitan Repository Refactoring Proposal

### Description

In order to support concurrent development of multiple projects, the repository needs to be refactored to account for IP and infrastructure versions.

The following slide deck presents [one such proposal](https://docs.google.com/presentation/d/1UMiN7qjHKrQ5Ylkuxq0TWeC6vbg_QMgAOmqSSUzhaFg/edit?usp=sharing).

@moidx
@cfrantz
@johannheyszl
@GregAC
@vogelpi
@andreaskurth

Start by reviewing the linked slide deck and the current repository organization. The proposal concerns supporting concurrent development through versioning IP and infrastructure, but it does not name files, tests, or a concrete implementation boundary. Done would require an agreed repository refactoring plan and its implementation across the affected structure.

Written by the indexing model from the issue text.