`Unrecognized field 'cpp-options'` in cabal.project

**Describe the bug**
cabal-install doesn't seem to recognize the `cpp-options` I have configured in my `cabal.project`.

**To Reproduce**
Steps to reproduce the behavior:

1. Checkout `unordered-containers` at https://github.com/haskell-unordered-containers/unordered-containers/commit/c481228ed539aeeb4b19d11384a14a2831cd4b84

2. Add the following `cabal.project`

```
packages: .

package unordered-containers
cpp-options: -DBENCH_containers_Map
```

3.

```
$ cabal build
Warning: /home/simon/src/unordered-containers/cabal.project: Unrecognized
field 'cpp-options' on line 4
```

**Expected behavior**
The CPP options are recognized and applied.

**System information**

```
$ cabal --version
cabal-install version 3.6.2.0
compiled using version 3.6.2.0 of the Cabal library
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.10.7
```