Feature flag to error if multiple versions of sentry-core are found

[Diggsey]

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I've used sentry quite a bit, and a very common issue is that you end up with two different versions of sentry-core in your application, which results in errors being silently dropped! This issue is really hard to debug because some errors come through just fine - it just depends which crate is reporting the error.

What I have done for our latest application is add a build.rs which reads the project-wide lockfile, and raises an error if multiple versions of sentry-core are found.

I think this functionality would be great to add directly to sentry-core's build script, enabled behind a feature flag, and for use of this feature flag to be recommended to application authors in the documentation.