[RFC]: Over-Arching Spec for BuildXL and other build system integration with Lage

[dannyvv]

In progress

Overview

We like to add support to lage for external build scheduler.
I'm using this issue as an overview and will create

There are larger build systems that can embed other build systems like BuildXL.
Modern build systems provide great features like sandboxing to ensure dependencies and outputs are declared to what actually happens. With reliable inputs and outputs one can distribute and cache builds reliably to increaes performance.
I want to extend Lage to allow for the specification.
I believe this can be done through a few updates / extensions to Lage.

These are the proposals in-order to consume:

1. Issue: [RFC]: Export all fields in Lage Info #815
   PR: Add extra info to lage info for #815 #836
2. Issue: [RFC]: Target settings support in package.json #817
3. Issue: [RFC]: Pipeline definition merging #816
4. Issue: [RFC]: Support inputs/output paths relative to different roots #818

And a few bugfixes

1. Fix working on Lage repo from Windows box by forcing LF line endings #807
2. Fix build if you clone lage repo in a 'src' folder #806
3. Add build:selfhost target for quick validation #835