Don't pass HscEnv to functions in the Hsc monad

`Hsc` is a reader monad in `HscEnv`. Several functions in HscMain were
taking parameters of type `HscEnv` or `DynFlags`, and returning values
of type `Hsc a`. This patch removes those parameters in favour of asking
them from the context.

This removes a source of confusion and should make refactoring a bit
easier.

Test Plan: ./validate

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D4061