Don't rely on tool for build-integration of svcutil.xmlserializer

[dasMulli]

Right now svcutil.xmlserializer uses the MSBuild <Exec> task to run a project-local tool.

This is a bit dangerous because it cannot work when multiple csproj files are in the same directory.
Also, project-local tools are deprecated by the global tools feature so it feels unnatural for users to use a deprecated feature for setting up xmlserializer integration.

Consider locating the executable from the .targets file (e.g. based on $(MSBuildThisFileDirectory)) to remove the need for the indirect lookup via the CLI.

[huanwu]

@dasMulli Thanks for your feedback. We'll take a look.