warn on duplicated value in `drom.toml` package section

[zapashcanon]

If a value is in a package.toml we should warn if an equal one is added in a drom.toml file package section.

E.g. when kind = "library" is present in both it should only be in the package.toml file. This should help to catch things like this one.

[lefessan]

The reason why we don't do it for now is that, during project creation, the drom.toml is supposed to overwrite options in package.toml. Indeed, we don't want that during standard updates. An idea would be to pass a ~should_warn flag that is enabled normally and disabled just during drom new