Use Project.ext as a fallback for property lookups

[isXander]

What

I have made WithPropertyLookup source from a configuration-time snapshot value
of Project.ext as a fallback for property lookups, when providers.gradleProperty() has no value.

Why

This change allows for programmatic configuration of NeoGradle conventions.

Such uses include:

• Plugins that apply before NeoGradle may set conventions required for NeoGradle to correctly behave in conjunction with it.
• build-logic conventions plugins may set the same set of conventions across all applied projects.

[neoforged-pr-publishing]

• Publish PR to GitHub Packages

[marchermans]

I think we can improve this a little bit.

[lukebemish]

This suggestion breaks project isolation, and current Gradle work seems to be moving heavily away from project properties and especially from project.ext n general (I'll try and find the recent issues/PRs when I have a chance). I am heavily opposed to this suggestion.

Use of project properties in general is a bad idea. In addition to breaking project isolation in certain scenarios when loaded from subprojects (see: arch loom, for details), this whole scenario makes a bunch of logic now order-dependent that was entirely declarative before.

If the goal is simply to be able to configure them per-project -- you should prefer having a Property<Boolean> extension field somewhere that is convention-ed to the gradleProperty. My understanding is that most of these fields already are? If they need to be loaded too early for that -- a Settings extension to configure values on the project extensions would likely suffice.