Add a z-empty.config lexically sorted last

cabal-testsuite/PackageTests/ProjectImport/DedupUsingConfigFromSimple/cabal.project

@@ -1,4 +1,5 @@
 packages: no-pkg-dir
+import: z-empty.config
 import: an-extra.config
 import: an-extra.config
 import: a-very-extra.config

cabal-testsuite/PackageTests/ProjectImport/DedupUsingConfigFromSimple/cabal.test.hs

@@ -17,6 +17,7 @@ main = cabalTest . recordMode RecordMarked $ do
       \ .*a-very-extra\\.config(\n|\r\n) \
       \ .*an-extra\\.config(\n|\r\n) \
       \ .*with-ghc\\.config(\n|\r\n) \
+      \ .*z-empty\\.config(\n|\r\n) \
       \ .*https://www.stackage.org/lts-21.25/cabal.config(\n|\r\n)"
     out
 

cabal-testsuite/PackageTests/ProjectImport/DedupUsingConfigFromSimple/z-empty.config

@@ -0,0 +1 @@
+-- This file is intentionally empty, just this comment.

changelog.d/pr-10546

@@ -12,14 +12,17 @@ build failure. We can trigger that message by using a non-existant package in
 the project, "no-pkg-dir".
 
 If an import is repeated in a `.project` or `.config` file it only imported once
-but if the same import is made from an imported file then it is repeated in the
-message.
+but if the same import is made from an imported file then it was being repeated
+in the message. Additional problems were not showing the project first and
+mixing configuration files and URIs together.
 
 * The test set up:
 
     ```
     $ cat cabal.project
+    cat cabal.project
     packages: no-pkg-dir
+    import: z-empty.config
     import: an-extra.config
     import: an-extra.config
     import: a-very-extra.config
@@ -34,6 +37,9 @@ message.
     $ cat a-very-extra.config
     import: https://www.stackage.org/lts-21.25/cabal.config
     import: https://www.stackage.org/lts-21.25/cabal.config
+
+    $ cat z-empty.config 
+    - This file is intentionally empty, just this comment.
     ```
 
 * Before the fix:
@@ -47,6 +53,7 @@ message.
     - https://www.stackage.org/lts-21.25/cabal.config
     - https://www.stackage.org/lts-21.25/cabal.config
     - https://www.stackage.org/lts-21.25/cabal.config
+    - z-empty.config
     The following errors occurred:
     - The package location 'no-pkg-dir' does not exist.
     ```
@@ -59,6 +66,7 @@ message.
     - cabal.project
     - a-very-extra.config
     - an-extra.config
+    - z-empty.config
     - https://www.stackage.org/lts-21.25/cabal.config
     The following errors occurred:
     - The package location 'no-pkg-dir' does not exist.