Basic smoke test for cabal sdist --list-sources

[quasicomputational]

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[quasicomputational]

According to my debugging putStrLn, the contents of --list-sources on Windows is this (note mixed slashes):

.\Main.hs
data\blah/a.dat
extra-doc/blah/a.tex
.\list-sources.cabal
extra-src/blah/a.html

Since the globstar patch rewrites the affected code, I'm going to roll up this PR into #5284 and close this one and see if I incidentally fixed it there.