Add handling of ambiguous package definitions within the same dir.

[obi1kenobi]

Instead of corrupting our data and possibly raising an unrelated error, cargo-semver-checks now explicitly checks if any of the package names in the same directory tree are defined by more than one manifest, and raises an appropriate error:

error: package `example` is ambiguous: it is defined by in multiple manifests within the root path .

defined in:
  ./Cargo.toml
  ./nested/Cargo.toml