cabal.project.local: Warn if referencing *different* branches of different subdirectories of same repository

**Describe the feature request**

It's quite common to have multiple packages in the same repo:
**cabal.project.local:**
```
source-repository-package
type: git
location: https://github.com/benjaminweb/selda
subdir: selda
tag: streaming

source-repository-package
type: git
location: https://github.com/benjaminweb/selda
subdir: selda-sqlite
tag: main
```
should raise a warning like:

`Warning: You are referencing different branches of the same repo with different subdirectories. If you are encountering weird compiler errors, change them to the same branch.`

**Additional context**
_What made you ask for this functionality?_
Spent quite some time to realise that I've sourced two subdirectories of the same repo with different branches.

_How would the feature benefit you and other users?_
A warning makes me instantly aware of the cause of the code not compiling instead of spending quite some time searching for the culprit.