initial config for subproject pkg customization

[loosebazooka]

So this is the setup for #10, it is not actually in use in sbom generation code yet. I still think it's the right thing to do for addition configuration of projects.

I think one can still do this from a single build file using allprojects/subprojects/project(":xyz") but I think build-logic is the right place to configure this across a ton of projects if it needs to be.

[loosebazooka]

I think we'll hold off on this till after the 1st release... this requires a few more followup prs.

[eygraber]

re allprojects/subprojects/project(":xyz"): keep in mind that Project Isolation is going to come at some point which will not allow that kind of functionality.

You should note that these constraints are not complete and more will be added over time. Also, it is likely that certain APIs are missing that are necessary for plugin authors to solve their use cases in a way that does not require direct access between projects. An important goal of this experiment is to identify these missing APIs.

At this stage, we do not recommend that you change your plugin to be somehow "project isolation compatible", as there is no stable definition of what this means yet, nor are all the necessary APIs in place to allow this. For now our aim is simply to understand that problems that are caused by adding the isolation. As the experiment continues and we refine the constraints and add new APIs, this will change and we will start encouraging plugin authors to try the new APIs.

[loosebazooka]

I'm going to rebase and merge this. And will work on integrating this data into a document in a followup pr