unify the setting of version + package metadata in the 461 and .net core builds of Paket

[baronfel]

Closes #4082

The merged .Net Framework 4.6.1 build of paket.exe used the 'raw' published binaries build from the publish step of the build script, which themselves used the raw outputs of the build step of the build script. This build step didn't assign version numbers to the version msbuild properties, and so that now-required metadata was no longer present.

This MR therefore

• ensures that versioning information is set at build-time, (and implicitly publish-time), not just pack-time
• tells ILRepack to migrate over attributes as well when doing the merge, so that the assemblyVersion is maintained on the merged binary

Verification of this can be seen by using ilspy or another decompiler to look at the AssemblyInformationalVersion attributes of the tools/paket.exe and tools/netcoreapp2.1/paket.dll files, which both should be 6.1.0 + a build hash after this change.

[forki]

this is what I get locally

[baronfel]

There's a hard coded test that I'm not sure has value anymore now that we're leaning on standard .net versioning metadata. I can look into it and see if there's a way to keep it to verify end to end, while not remaining hard coded.