Avoid always rerunning GetModificationTime for interface files too

[pepeiborra]

Extends the changes introduced in #1487 to interface files.

To verify:

[nix-shell:~/scratch/ide]$ cabal exec cabal run ghcide-bench -- -- --example-package-name Cabal --example-module Distribution/Simple.hs --example-package-version 3.0.0.0 -s "edit" --samples 1 --shake-profiling /tmp/ghcide --no-clean
Up to date
Config {verbosity = Normal, shakeProfiling = Just "/tmp/ghcide", otMemoryProfiling = Nothing, outputCSV = "results.csv", buildTool = Cabal, ghcideOptions = [], matches = ["edit"], repetitions = Just 1, ghcide = "ghcide", timeoutLsp = 60, example = GetPackage {exampleName = "Cabal", exampleModules = ["Distribution/Simple.hs"], exampleVersion = Version {versionBranch = [3,0,0,0], versionTags = []}}}
starting test
Setting up document contents took 7.18s
Running edit benchmark
0.47s
name | success | samples | startup | setup | userTime | delayedTime | totalTime
---- | ------- | ------- | ------- | ----- | -------- | ----------- | ---------
edit | True    | 1       | 8.67s   | 0.00s | 0.47s    | 0.00s       | 0.47s    

[nix-shell:~/scratch/ide]$ open /tmp/ghcide/ide-20210306-135108-00006.html