Downgrade NoLibraryFound from an error to a warning

This makes Setup copy/install succeed if there's
nothing to do because the package doesn't contain
a library or executable.

This allows downstream users of Cabal to avoid having to
add workarounds for this edge case.

Resolves #6750

Cabal/src/Distribution/Simple/Errors.hs

@@ -47,7 +47,8 @@ data CabalException
   | EnableBenchMark
   | BenchMarkNameDisabled String
   | NoBenchMark String
-  | NoLibraryFound
+  | -- | @NoLibraryFound@ has been downgraded to a warning, and is therefore no longer emitted.
+    NoLibraryFound 
   | CompilerNotInstalled CompilerFlavor
   | CantFindIncludeFile String
   | UnsupportedTestSuite String

Cabal/src/Distribution/Simple/Install.hs

@@ -147,7 +147,7 @@ install_setupHooks
 
       checkHasLibsOrExes =
         unless (hasLibs pkg_descr || hasForeignLibs pkg_descr || hasExes pkg_descr) $
-          dieWithException verbosity NoLibraryFound
+          warn verbosity "No executables and no library found. Nothing to do."
 
 -- | Copy package global files.
 copyPackage

changelog.d/issue-6750

@@ -0,0 +1,13 @@
+synopsis: Make Setup copy/install succeed when there's no executable or library
+packages: Cabal
+prs: TODO
+issues: #750
+
+description: {
+  Historically the Setup copy and install steps would fail if the package didn't
+  contain an executable or library component. In this case there's nothing to do.
+
+  This required workarounds for downstream users of Cabal to handle this edge case.
+  Now that this error has been downgraded to a warning, Cabal will succeed if 
+  there's nothing to do.
+}