Enable <PublishedSingleFileName> to be set in .csproj

[jnm2]

I'd like to be able to control the published single file name independently from the assembly name. I can't set the assembly name to the desired published single file name because it would conflict with a library assembly name in the same project. However, the published single file name can be set to anything without causing a conflict, as evidenced by testing after renaming it to the desired file name after publishing.

It seems setting <PublishedSingleFileName>Xyz.exe</PublishedSingleFileName> in the .csproj is very close to working:

sdk/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Publish.targets

Lines 1069 to 1072 in f498215

	<PropertyGroup>
	<PublishedSingleFileName>$(AssemblyName)$(_NativeExecutableExtension)</PublishedSingleFileName>
	<PublishedSingleFilePath>$(PublishDir)$(PublishedSingleFileName)</PublishedSingleFilePath>
	</PropertyGroup>

We just need to change one line to follow the standard MSBuild "provide default if not already set" pattern:

    <PropertyGroup>
      <PublishedSingleFileName Condition="'$(PublishedSingleFileName)' == ''">$(AssemblyName)$(_NativeExecutableExtension)</PublishedSingleFileName>
      <PublishedSingleFilePath>$(PublishDir)$(PublishedSingleFileName)</PublishedSingleFilePath>
    </PropertyGroup>

Alternatively, if it would be desirable to use a different csproj property which specifies only the name part without the .exe file extension, a new property could be introduced or PublishedSingleFileName could be refactored to not include the .exe part.

Happy to send a PR.