DynFlags: Introduce -show-mods-loaded flag

This flag reintroduces the verbose module name output produced by GHCi's
:load command behind a new flag, -show-mods-loaded. This was originally
removed in D3651 but apparently some tools (e.g. haskell-mode) rely on
this output.

Addresses #14427.

Test Plan: Validate

Reviewers: svenpanne

Reviewed By: svenpanne

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D4164

(cherry picked from commit 8613e61d)