Backport infrastructure cleanup to share sources and project references (#4513)

.gitignore

Directory.Build.targets

build/artifacts.props

build/repo.props

src/MetaPackages/korebuild-lock.txt

src/ServerTests/korebuild-lock.txt

src/Servers/HttpSys/.gitignore

src/Servers/HttpSys/NuGetPackageVerifier.json

src/Servers/HttpSys/test/Directory.Build.props

...est/Microsoft.AspNetCore.Server.Kestrel.Core.Tests.csproj

src/Shared/Buffers.Testing/Directory.Build.props

src/Shared/Diagnostics/Directory.Build.props

src/Shared/Directory.Build.props

...hared/Hosting.WebHostBuilderFactory/Directory.Build.props

src/Shared/HttpSys/Directory.Build.props

....Tests/Microsoft.AspNetCore.Http.Connections.Tests.csproj

src/Templating/korebuild-lock.txt