### Short description
When using `TuistGenerator` as a library, the generator component performs several actions:
- Loading of the graph
- Linting the graph
- XcodeProj generation
- Writing generation artifacts to disk
- Post generation actions
While convenient it doesn't offer flexibility to perform any custom linting or post processing on the generated XcodeProj representations or to selectively leverage some of those actions without the others.
### Solution
Based on a few early prototyping rounds on [tuist-labs](https://github.com/tuist/tuist-labs/tree/prototype/pipelines) and discussions on modularising Tuist even further, we can split the responsibilities of the generator to provide more flexibility and simplicity.
- Update the generator to return descriptors (it doesn't directly perform any operations - it's side effect free)
- Create a dedicated component to write those descriptions to disk as well as perform any required side effects
- Move some of the interactors that performed side effects out of the xcodeproj generation to help group them (e.g. CocoaPods / Swift PM)
- Create a new top level component to orchestrate all those steps to achieve the previous behaviour
_**Note:** These changes should have no impact on end users of Tuist_
### Implementation
- [x] Prototype `Descriptors` by having existing components return descriptors instead of performing actions (initially as duplicate methods for testing/prototyping)
- [x] Integrate a prototype of a high level `ProjectGenerator` that performs all the previous tasks with the `tuist generate` command
- [x] Verify acceptance tests don't flag any regressions
- [x] Delete duplicate methods (the non-descriptor) variants and update their corresponding unit tests
- [x] Integrate the updated generator with the other tuist commands
- [x] Come up with better component names to clarify their purpose _(current names were temporary to avoid name collisions however could be confusing)_
- [x] Update change log
### Test Plan
- Verify all acceptance tests pass
- Test out running `tuist generate` on any of the fixtures
- Building Tuist when opening it Natively in Xcode (double clicking the Package.swift) yields a few buidl errors
- This is due to missing explicit dependencies between targets
Test Plan:
- Open the Package.swift to launch it in Xcode
_Note: even with this fix it doesn't seem the ProjectDescription framework/dylib is being built correctly in this mode, the current workaround is to continue to use `swift package generate-xcodeproj`_
Pedro Piñera Buendía