main-is: inside two-way if/else: Ambiguous values for modulePath field

**Describe the bug**

cabal file containing
```
executable foo-bar
if flag(wat)
main-is: bar-foo.hs
else
main-is: foo-bar.hs
```
is rejected with
```
Ambiguous values for modulePath field: '"bar-foo.hs"' and '"foo-bar.hs"'
CallStack (from HasCallStack):
error, called at src/Distribution/Types/UnqualComponentName.hs:128:7 in Cabal-syntax-3.11.0.0-inplace:Distribution.Types.UnqualComponentName
```

**To Reproduce**

```
$ cat foo.cabal
cabal-version: 3.0
name: foo
version: 0

flag wat
default: False
manual: True

executable foo-bar
if flag(wat)
main-is: bar-foo.hs
else
main-is: foo-bar.hs
hs-source-dirs: src
build-depends: base

$ cabal install
```

**Expected behavior**
builds

**System information**

```
6.6.12-200.fc39.x86_64 x86_64 GNU/Linux

cabal-install version 3.11.0.0
compiled using version 3.11.0.0 of the Cabal library

ghc-9.6.3
```

**Additional context**

with cabal-install 3.10.2.1, the problem can be repaired with indenting `else`, see. https://github.com/ucsd-progsys/liquidhaskell/issues/2258#issuecomment-1910968091 but cabal-install-3.11 will reject that with
```
Warning: foo.cabal:12:4: invalid subsection "else"
Warning: foo.cabal:12:4: Inconsistent indentation. Indentation jumps at lines 12
```

Start with the provided foo.cabal reproducer and inspect the parsing path around src/Distribution/Types/UnqualComponentName.hs:128, where the ambiguity is reported. Run cabal install with cabal-install 3.11 and confirm that the two-way if/else main-is values are accepted without indentation warnings; done means the example builds as expected.

Written by the indexing model from the issue text.