`.cabal` file "flag used but not defined" error message has inaccurate source location.

In a .cabal file like this:

```cabal
cabal-version: 2.4
name: hls-test-utils
version: 2.10.0.0
build-type: Simple

library
hs-source-dirs: src
build-depends:
, base
if flag(unix)
build-depends: unix < 2.9

default-language: GHC2021
```

I get the error message:

```
/home/hugin/Documents/haskell/hls/hls-test-utils/hls-test-utils.cabal:0:0: These flags are used without having been defined: unix
```

Cabal should report exactly where the flag is used but not defined.

This looks especially silly in HLS:


Reproduce the diagnostic with the .cabal example from the issue and trace how the undefined flag in `if flag(unix)` is reported. Done means the error identifies the location where the flag is used rather than reporting `:0:0`.

Written by the indexing model from the issue text.