Why must both PYTHON_JULIACALL_PROJECT and PYTHON_JULIACALL_EXE be set together?

I wanted to set my `PYTHON_JULIACALL_PROJECT` manually because i wanted to manage my julia dependencies manually, so i could use them both in julia (for testing) and via juliacall.
And because i didn't want them to be in my conda directory as that doesn't get version controlled and i wanted the manifest to get version controlled

But I didn't want to set my `PYTHON_JULIACALL_EXE` because I do not know where julia wil be installed on any of my collaborators machines and am happy to let them be worked out by juliapkg.

But i tried this and then got an error on `import juliacall`
```
Exception: Both PYTHON_JULIACALL_PROJECT and PYTHON_JULIACALL_EXE must be set together, not only one of them.
```

Start at the import juliacall entry point and trace how PYTHON_JULIACALL_PROJECT and PYTHON_JULIACALL_EXE are validated. Check how juliapkg resolves Julia when the executable variable is unset. Done means a manually managed project can be used without specifying the executable, while existing paired settings continue to work.

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