Do not override existing `pyproject.toml` keys during conversion

### What is the problem or limitation you are having?

When an existing project is converted, keys should not be added to `[tool.briefcase]` when they already exist in `[project]`. For instance, if `version` is already specified in `[project]`, do not add `version = "0.0.1"` to `[tool.briefcase]`. This should be considered for all PEP-621 `pyproject.toml` keys.

### Describe the solution you'd like

Omit `[tool.briefcase]` keys from `pyproject.toml` that are already represented in `[project]`.

### Describe alternatives you've considered

Manually remove overriding keys from `[tool.briefcase]` post-conversion.

### Additional context

_No response_

Start by locating the conversion logic that writes pyproject.toml, then compare PEP-621 keys in [project] with keys generated under [tool.briefcase]. Convert an existing project containing representative [project] keys and verify that already represented keys are omitted from [tool.briefcase] without removing unrelated configuration.

Written by the indexing model from the issue text.