protobuf-net / protobuf-net

* better option handling
* detect incompatible enums at build; implements #736 as an analyzer
* check for redundant enum names, too
* stab at relative path processing on imports

code analysis pass; remove analyzer dependencies

* experimental .proto schema generator; requires .NET 5 SDK

* This (adding a different nuget package) is a crap solution to the SDK-targeting problem, but it is the best one I can think of

* add better option support

# Conflicts:
#	src/protobuf-net.BuildTools/protobuf-net.BuildTools.csproj

…N0015 if possible

initial stab at build tools (analyzers and generators); much more to do

The lookup of files uses a somewhat flawed `MakeRelativePath` method :
- it doesn't always use the builtin `Path.GetRelativePath` when available
- when not using `Path.GetRelativePath` (and only then), it requires that
  the `proto_path` ends with a directory separator.

This is part of what has been seen in #406.