Refactor/before import warning

[philderbeast]

Split from #10933, the refactoring and the sorting changes without the duplicate import by alternate paths detection, as requested by @ulysses4ever.

• Add a new module cabal-install/src/Distribution/Client/ProjectConfig/Import.hs and move some stuff there from cabal-install-solver/src/Distribution/Solver/Types/ProjectConfigPath.hs that was only ever used in cabal-install.

For messages, I added some doctests and ended up with two ways of comparing ProjectConfigPath.

-- | A comparison that puts projects first, URLs last and sorts the other paths
-- lexically.
compareLexicographically :: ProjectConfigPath -> ProjectConfigPath -> Ordering

-- | A comparison that puts projects first, URLs last and sorts the other paths
-- by putting longer paths after shorter ones as measured by the number of path
-- segments. If still equal, then sorting is lexical.
compareSegmentally:: ProjectConfigPath -> ProjectConfigPath -> Ordering

I'll squash commits before applying the merge label if this pull request is approved.

---

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[ulysses4ever]

Thanks a lot for splitting it --- it helped reviewing a lot! A couple nitpicks inline.

[ulysses4ever]

Is this change necessary? I dislike unrelated changes even if they are good changes on their own. But I won't block on this.

[philderbeast]

Yes it is. With +legacy-comparison both legacy and parsec parsers are emitting the untrimmedUriImportMsg message. This leads to a test failure:

UNEXPECTED FAIL: PackageTests/ProjectImport/UntrimmedImport/cabal.test.hs

The test expects only one instance of this error for each build:

cabal/cabal-testsuite/PackageTests/ProjectImport/UntrimmedImport/cabal.out

Lines 2 to 5 in e6ff0cc

	# cabal v2-build
	Warning: import has leading or trailing whitespace;
	'https://www.stackage.org/nightly-2024-12-05/cabal.config '
	imported by: trailing-space.project

Here's the test diff with the warning repeated:

$ cabal build all --enable-tests -j --semaphore --ghc-options="-Werror" \
  --project-file=cabal.validate.project

$ cabal run cabal-testsuite:cabal-tests -- \
  --with-cabal=./dist-newstyle/build/x86_64-linux/ghc-9.14.1/cabal-install-3.17.0.0/x/cabal/build/cabal/cabal \
  cabal-testsuite/PackageTests/ProjectImport/UntrimmedImport/cabal.test.hs
...
Actual output differs from expected:

# STDERR:
--- /tmp/cabal-testsuite-712126/cabal.dist/cabal.out.normalized 2026-05-04 14:40:41.311315596 -0400
+++ /tmp/cabal-testsuite-712126/cabal.dist/cabal.comp.out.normalized    2026-05-04 14:40:41.311270635 -0400
@@ -3,6 +3,9 @@
 Warning: import has leading or trailing whitespace;
   'https://www.stackage.org/nightly-2024-12-05/cabal.config '
     imported by: trailing-space.project
+Warning: import has leading or trailing whitespace;
+  'https://www.stackage.org/nightly-2024-12-05/cabal.config '
+    imported by: trailing-space.project
 Resolving dependencies...
 Build profile: -w ghc-<GHCVER> -O1
 In order, the following would be built:
@@ -12,6 +15,9 @@
 Warning: import has leading or trailing whitespace;
   'https://www.stackage.org/nightly-2024-12-05/cabal.config               '
     imported by: tabs-and-spaces.project
+Warning: import has leading or trailing whitespace;
+  'https://www.stackage.org/nightly-2024-12-05/cabal.config               '
+    imported by: tabs-and-spaces.project
 Resolving dependencies...
 Build profile: -w ghc-<GHCVER> -O1
 In order, the following would be built:
cabal.test.hs: Uncaught exception ghc-internal:GHC.Internal.Exception.ErrorCall:

FAIL

HasCallStack backtrace:
  throwIO, called at src/Test/Cabal/Monad.hs:533:24 in cabal-testsuite-3-inplace:Test.Cabal.Monad