Duplicate project imports slow down the build

When a project has duplicate imports the build is slower than it could be[^1].

To see this in action, I've created a `woops` project, the same as the `yops` one (the Y-forking one from #10508) but every `.project` or `.config` also imports `https://www.stackage.org/lts-21.25/cabal.config`. The former takes `12 s` to `cabal build --dry-run` and the later takes `200 ms`. If I add one import from stackage to the root of the `yops` project, its build takes `425 ms`.

```
$ ghc --numeric-version
9.4.8

$ time cabal run cabal-install:exe:cabal -- build --dry-run \
--project-file=cabal-testsuite/PackageTests/ConditionalAndImport/woops-0.project
Resolving dependencies...
Warning: this is a debug build of cabal-install with assertions enabled.
When using configuration from:
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- https://www.stackage.org/lts-21.25/cabal.config
- woops-0.project
- woops-2.config
- woops-4.config
- woops-4.config
- woops-6.config
- woops-6.config
- woops-6.config
- woops-8.config
- woops-8.config
- woops-8.config
- woops-8.config
- woops/woops-1.config
- woops/woops-3.config
- woops/woops-3.config
- woops/woops-5.config
- woops/woops-5.config
- woops/woops-5.config
- woops/woops-7.config
- woops/woops-7.config
- woops/woops-7.config
- woops/woops-7.config
- woops/woops-9.config
- woops/woops-9.config
- woops/woops-9.config
- woops/woops-9.config
- woops/woops-9.config
The following errors occurred:
- The package directory '.' does not contain any .cabal file.

________________________________________________________
Executed in 12.08 secs fish external
usr time 2.94 secs 0.00 micros 2.94 secs
sys time 0.45 secs 802.00 micros 0.45 secs
```

```
$ time cabal run cabal-install:exe:cabal -- build --dry-run \
--project-file=cabal-testsuite/PackageTests/ConditionalAndImport/yops-0.project
Warning: this is a debug build of cabal-install with assertions enabled.
When using configuration from:
- yops-0.project
- yops-2.config
- yops-4.config
- yops-4.config
- yops-6.config
- yops-6.config
- yops-6.config
- yops-8.config
- yops-8.config
- yops-8.config
- yops-8.config
- yops/yops-1.config
- yops/yops-3.config
- yops/yops-3.config
- yops/yops-5.config
- yops/yops-5.config
- yops/yops-5.config
- yops/yops-7.config
- yops/yops-7.config
- yops/yops-7.config
- yops/yops-7.config
- yops/yops-9.config
- yops/yops-9.config
- yops/yops-9.config
- yops/yops-9.config
- yops/yops-9.config
The following errors occurred:
- The package directory '.' does not contain any .cabal file.

________________________________________________________
Executed in 200.73 millis fish external
usr time 148.65 millis 0.00 micros 148.65 millis
sys time 52.03 millis 692.00 micros 51.33 millis

$ echo "import: https://www.stackage.org/lts-21.25/cabal.config" \
>> cabal-testsuite/PackageTests/ConditionalAndImport/yops-0.project

$ time cabal run cabal-install:exe:cabal -- build --dry-run \
--project-file=cabal-testsuite/PackageTests/ConditionalAndImport/yops-0.project
Warning: this is a debug build of cabal-install with assertions enabled.
When using configuration from:
- https://www.stackage.org/lts-21.25/cabal.config
- yops-0.project
- yops-2.config
- yops-4.config
- yops-4.config
- yops-6.config
- yops-6.config
- yops-6.config
- yops-8.config
- yops-8.config
- yops-8.config
- yops-8.config
- yops/yops-1.config
- yops/yops-3.config
- yops/yops-3.config
- yops/yops-5.config
- yops/yops-5.config
- yops/yops-5.config
- yops/yops-7.config
- yops/yops-7.config
- yops/yops-7.config
- yops/yops-7.config
- yops/yops-9.config
- yops/yops-9.config
- yops/yops-9.config
- yops/yops-9.config
- yops/yops-9.config
The following errors occurred:
- The package directory '.' does not contain any .cabal file.

________________________________________________________
Executed in 425.02 millis fish external
usr time 210.23 millis 0.00 micros 210.23 millis
sys time 49.17 millis 717.00 micros 48.45 millis
```

[^1]: #9933 tries to avoid this by making duplicate imports an error.