ghc-pkg: Consider .conf files when computing package db mtime
ghc-pkg: Consider .conf files when computing package db mtime

We can no longer use the mtime of the containing directory since it now
contains a lock file in addition to the .cache and .conf files.

Fixes #13375.

Test Plan: Validate on Windows

Reviewers: austin, arybczak

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3289

shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc