Open your project file with notepad and set this items to true
<IncludeAssemblyInVSIXContainer>true</IncludeAssemblyInVSIXContainer>
<IncludeDebugSymbolsInVSIXContainer>true</IncludeDebugSymbolsInVSIXContainer>
<IncludeDebugSymbolsInLocalVSIXDeployment>true</IncludeDebugSymbolsInLocalVSIXDeployment>
<CopyBuildOutputToOutputDirectory>true</CopyBuildOutputToOutputDirectory>
<CopyOutputSymbolsToOutputDirectory>true</CopyOutputSymbolsToOutputDirectory>