Unexpected project file semantics with conditionals

Project file semantics seem to be confusing/broken with conditionals.

This is especially problematic in light of project file imports where we can very well have some repetition and want things that come later as overrides.

# Reproducer

I've created a reproducer where the test fails to compile, but I'm disabling the tests in a convoluted way in the cabal project file.

These are all run on a linux system.

## With conditional

`cabal.project`:

```cabal
packages: ./cabal-project-repro.cabal

if !os(windows)
package *
Tests: True

package *
Tests: False
```

Running `cabal build` results in:

```
test/Main.hs:4:8: error: [GHC-88464]
Variable not in scope: puStrLn :: String -> IO ()
Suggested fix: Perhaps use ‘putStrLn’ (imported from Prelude)
|
4 | main = puStrLn "Test suite not yet implemented."
| ^^^^^^^
[2 of 2] Linking dist-newstyle/build/x86_64-linux/ghc-9.8.4/cabal-project-repro-0.1.0.0/x/filemonitor-test/build/filemonitor-test/filemonitor-test
Error: [Cabal-7125]
Failed to build test:cabal-project-repro-test from cabal-project-repro-0.1.0.0.
```

So the tests are actually built!

Reproducer: `git clone https://github.com/hasufell/cabal-project-repro.git --branch conditional`

## Without conditional

Just removing the os-conditional suddenly makes it succeed.

`cabal.project`:

```cabal
packages: ./cabal-project-repro.cabal

package *
Tests: True

package *
Tests: False
```

Reproducer: `git clone https://github.com/hasufell/cabal-project-repro.git --branch no-conditional`

## Explanation attempt

My guess is that the conditionals are resolved later and so actually come **last** when all the expressions are applied and resolved to packages. I don't think that's something a reader would expect to happen, just looking at the ordering of expressions in the file.

Start with the conditional and non-conditional cabal.project files in the linked reproducer branches, then run cabal build on Linux to compare their behavior. Trace how the package and Tests settings are ordered and resolved; done means the conditional configuration follows the documented or expected override semantics and the reproducer builds accordingly.

Written by the indexing model from the issue text.