Fix #8756: doc: `extra-source-files` are tracked by `cabal build` (#8765)
Changing a file from `extra-source-files` causes `cabal build` to
reevaluate the build plan, judging from the following behavior.
```console
$ cabal build
...
$ cabal build
Up to date
# Make a change to any of the extra-source-files, e.g. Foo.txt
$ cabal build
...
In order, the following would be built (use -v for more details):
- ... (lib) (file Foo.txt changed)
...
```
One notes here that the 3rd `cabal build` does not just output "Up to date".
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit
fdcf77b2e6d70a665ef4c2fc0c08bd850ab8114b)