Proposed Changes

There are a couple of problems with RabbitMQ.Client.csproj that this PR fixes:

• The library's license was specified in a file. Since the license is a combination of known open-source licenses, it is now specified as an SPDX license expression, helping readability for both humans and machines.
• The project was publishing its debug symbols both in the main .nupkg file, and in a separate .snupkg file. There is no reason to do both; the symbols are now included only in the .snupkg, saving some tens of kilobytes from the .nupkg's size. If there are compatibility concerns with older IDEs I think we should discontinue the .snupkg instead for the sake of reducing duplication.
• The project was publicly referencing two redundant packages; the one contains analyzers that do not need to be public, and the other contains the Span type and its friends for frameworks that don't already have it; .NET Core does not need it.

Types of Changes

• Bug fix (non-breaking change which fixes issue N/A)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works (not applicable)
• I have added necessary documentation (if appropriate) (not applicable)
• Any dependent changes have been merged and published in related repositories (not applicable)

Further Comments

N/A