dotnet pack does not support arbitrary configurations

PJ allowed passing in arbitrary configurations, and would output to the appropriate bin/{Configuration} subdirectory. CSProj fails with:

error : The OutputPath property is not set for project 'TestLibraryWithConfiguration.csproj'.  Please check to make sure that you have specified a valid combination of Configuration and Platform for this project.  Configuration='Test'  Platform='AnyCPU'.  You may be seeing this message because you are trying to build a project without a solution file, and have specified a non-default Configuration or Platform that doesn't exist for this project.

@nguerrera how do we enable custom configurations?

[nguerrera]

The biggest challenge here is deciding on the settings applied to such a configuration. We can go ahead and let the build proceed to an arbitrary directory. This trades off any enforcement against misspelling of configurations. So I say ‘releease’ or ‘deebug’, and get something rather arbitrary instead of an error.

We’ve talked about this lenient approach and it’s the cheapest way to unblock adding a new configuration in VS, which is currently broken, but just letting things proceed wouldn’t actually give the right behavior. In VS, when you create a new configuration, you say from which other configuration you want settings to inherit. If we just set the output path and nothing else, you wouldn’t inherit appropriately.

So we have to decide two things:

1. Do we want enforcement of configurations or is it garbage-in-garbage-out?
2. Do we support this ‘make a new configuration X with settings seeded from Y’ experience that has always existed in VS? If so, what’s the mechanism for triggering this behavior in a new world where there needn’t be a property group for every Configuration|Platform combination? If not, we need to track VS changes.

In short, this isn’t as simple as “just always set the output path”. There are consequences.

[livarcocc]

Created a copy of this in the SDK: https://github.com/dotnet/sdk/issues/328