I had a similar issue, in my case I wanted to pack C# code using ExcelDNA tools that come with the package. The post build event was added as part of the package install, but depended on the package folder been located in the solution folder, as you say. Porting the code to Net Standard we realised that the new version of nuget cannot deal with this specific issue of post build events which require the package tools to be in some folder relative to the solution or project. So to directly answer your question see here.
However you may not find that very useful if you want to do a post build event from the solution? If you are using jenkins et al and have a seperate step for packaging then the above should work fine.
There are a couple of ways around it. Most simply you can add the tools to source control, and then manually edit the post-build events. Depending on how you feel about that.
Secondly you can force nuget to resotre the packages locally and not in the system wide cache folder. You can do this through the Nuget.Config file. I have not got the specifics to hand, but if you cannot figure it out I can look in my old code.
