Make jetbrains.annotations private

src/Directory.Build.props

@@ -4,6 +4,7 @@
 
   <PropertyGroup Label="Package">
     <Product>CAP</Product>
+    <LangVersion>8</LangVersion>
     <Authors>ncc;savorboard</Authors>
     <RepositoryUrl>https://github.com/dotnetcore/CAP</RepositoryUrl>
     <RepositoryType>git</RepositoryType>
@@ -24,6 +25,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All"/>
+    <PackageReference Include="JetBrains.Annotations" Version="2019.1.3" PrivateAssets="All" />
   </ItemGroup>
 
 </Project>

src/DotNetCore.CAP/DotNetCore.CAP.csproj

@@ -10,7 +10,6 @@
   </PropertyGroup>
   
   <ItemGroup>
-    <PackageReference Include="JetBrains.Annotations" Version="2019.1.3" />
     <PackageReference Include="Microsoft.Extensions.Hosting.Abstractions" Version="3.0.0" />
     <PackageReference Include="Microsoft.Extensions.Options" Version="3.0.0" />
     <PackageReference Include="Newtonsoft.Json" Version="12.0.3" />