Treat TargetFramework(s) values as aliases

[emgarten]

NuGet should treat the values from TargetFramework and TargetFrameworks as aliases.

When setting the property TargetFramework=(alias) the actual target framework value can be evaluated from TargetFrameworkIdentifier and TargetFrameworkVersion.

See for examples and more details: dotnet/project-system#1938

Update by @nkolev92

NuGet in 5.8 and .NET 5.0 SDK now treats TargetFramework as an alias, however duplicate framework entries are not supported yet (ex. You can't target .NETCoreApp,Version=3.0 twice.)

[davkean]

Yes, see this guideline I just wrote: https://github.com/dotnet/project-system/blob/master/docs/repo/coding-conventions.md#data

[natidea]

RE: dotnet/project-system#615
Nominate currently passes both TF and TFM. TF is the key used to identify each TargetFrameworkInfo object (though the api calls it "TargetFrameworkMoniker"), and TFM is stored in the Properties bag. I'll close dotnet/project-system#615 since we are already passing all the info NuGet needs to make this switch.

[nkolev92]

@rrelyea we should discuss this and tackle it for .NET 5.0.