Always generate ModDetails from ModIface

This vastly reduces memory usage when compiling with --make mode, from about 900M when compiling Cabal to about 300M.

As a matter of uniformity, it also ensures that reading from an interface performs the same as using the in-memory cache. We can also delete all the horrible knot-tying in updateIdInfos.

Goes some way to fix #13586 (closed)

Here for CI at the moment. Doesn't seem to noticeably regress --make performance.