Reference Mono.Cecil using a PackageReference

[sharwell]

The current submodule and bundling approach causes problems for consumers of ICSharpCode.Decompiler due to the embedding of a "non-standard" build of Mono.Cecil.dll. This pull request modifies the references to use PackageReference instead, and declare a NuGet dependency on the Mono.Cecil package on NuGet.

For this pull request, I did not remove the submodule itself. It's possible that developers have local branches in the submodule and would like to continue working with those branches, so I decided to leave that decision for a future pull request.

This pull request is very similar to #998. Specific differences are:

1. This pull request leaves the cecil submodule in place, and updates the commit reference to match the commit we're referencing as a NuGet package
2. This pull request leverages transitive <PackageReference> in a few places, so there are fewer explicit references to Mono.Cecil
3. This pull request updates the NuGet package specification for ICSharpCode.Decompiler to declare the dependency on Mono.Cecil

[siegfriedpammer]

@sharwell did you see #998?

[sharwell]

@siegfriedpammer Yes, the differences as compared to #998 are listed above. This pull request is a slightly different approach to the same goal.