The project information (corresponds to a single elm.json file).
Holds more information though - the data in the type parameter holds all the
parsed modules etc. - what every stage needs. See ProjectFields of the various
AST stages.
Each AST stage has its own project fields - that's what the r parameter is.
Eg. on the frontend we have program : Modules Frontend.Expr
and on the backend we have graph : Backend.Graph.
Each AST stage has its own project fields - that's what the
rparameter is. Eg. on the frontend we haveprogram : Modules Frontend.Exprand on the backend we havegraph : Backend.Graph.