Packing error when using platform specific TFMs.

[AraHaan]

Issue Details

Describe the bug
A clear and concise description of what the bug is.
Packagine error when trying to package a project containing an platform specific TFM.

The issue is that it does it check if it's a platform specific TFM and then append the Platform Version number that it should target.

To Reproduce
Steps to reproduce the behavior:

Just have a project that contains net5.0-windows or any other platform target, notice that when packing that the packing will fail for sure.

Expected behavior
A clear and concise description of what you expected to happen.
For the packing to work correctly.

Actual behavior
The actual result.
Packing errors out.

Screenshots
If applicable, add screenshots to help explain your problem.

Version used
The version affected by this issue (Can also be "master" for code in latest master branch but reference commit then if possible).
Latest Nuget package.

Additional context
Add any other context about the problem here.

[AraHaan]

Checklist for https://github.com/dotnet/designs/blob/main/accepted/2020/net5/net5.md#design, https://github.com/dotnet/designs/blob/main/accepted/2021/net6.0-tfms/net6.0-tfms.md#design

• net5.0 | net6.0
• net5.0-windows | net6.0-windows
• net5.0-android | net6.0-android
• net5.0-ios | net6.0-ios
• net5.0-macos | net6.0-macos
• net6.0-maccatalyst (added in .NET 6)
• net5.0-tvos | net6.0-tvos
• net5.0-watchos (removed in .NET 6)