Issue Description:

(I am observing this with the package manager, but I imagine the issue is more widespread.)

Module names seem to support Unicode. But it is not consistent. Some names work just fine:

mkdir Εφ
cd Εφ
swift package init
swift test

But others break:

mkdir Εφαρμογή
cd Εφαρμογή
swift package init
swift test

The tests pass, but the module can only be imported with Εφαρμογη_, which does not match the manifest. In this case it seems to be triggered by a combining scalar.

On macOS everything is in NFD at this point, due to the original directory name. What if we try to work around it by changing the manifest entries to NFC so the tonos is not a separate scalar? ("Εφαρμογ\u{3AE}")

No dice; regardless of whether the import statement itself is in NFD or NFC, this happens:

$ swift test
[...]/Εφαρμογή/Tests/ΕφαρμογήTests/Εφαρμογη_Tests.swift:2:18: error: cannot load module 'Εφαρμογή' as 'Εφαρμογή'
@testable import Εφαρμογή

And looking inside the .build directory, I see internally inconsistent entries like this:

[...]/Εφαρμογή.build/Εφαρμογη_.swift.o