Remove use of .pkgproj from repos outside of dotnet/runtime

[akoeplinger]

• This issue is blocking
• This issue is causing unreasonable pain

See discussion in #5818 (comment).

https://github.com/dotnet/llvm-project and https://github.com/dotnet/icu are using .pkgproj to produce nuget packages with (probably non-ideal) logic copied from dotnet/runtime. Those usages should be removed.

/cc @ViktorHofer @ericstj

[ericstj]

I don't think we're going to go into other folks repositories and remove their use of pkgproj. We can collaborate to suggest alternatives, but I don't think we should be tracking this here,.

[akoeplinger]

Well I don't really care where it's tracked and I'm kinda maintaining the packaging for these repos :)

[akoeplinger]

Just to set expectations, I will need help with replacing them as I have no idea how it's supposed to be done "the right way" 😄

[ericstj]

Don’t use PKGProj :). Use publicly documented pack target that’s integrated with the SDK.