* Run libraries package testing on build agent

instead of on Helix as with recent changes the entire package testing
doesn't take more than 2 minutes. Helix created a work item per package
test even though it only took seconds and the average wait time for
a client was 10-15min.

Also cleaning up how the generated package test projects are restored
and incorporating a fix from Eric St John to not hard code the package
feeds.

* Expose useHelix argument

* Include NetCoreAppCurrent configs in packages

The NetCoreAppCurrent configurations were omitted from packages to avoid
ever growing packages. Now that we adhere to the support policy for
packages we don't need to exclude them anymore as the policy defines a
baseline for .NETCoreApp configurations.

This will remove an artificial difference when source building the
repository and also make it so that partner repositories which don't
depend on a transport package like windowsdesktop (winforms) receive
the very latest assets that are included in the shared framework as
well.

* Fix package validation for non exposed inbox libs

* Change package testing to use RuntimeTargets rather than RID-specific restore

* Upload binlogs for package testing

* Address feedback

Simplify outer build of package tests to just use InnerTargets extension point.

Use Helix's work-item isolated packages folder

Give a better name to binlogs

* Cleanup old suppressions

* Use ReferenceCopyLocalPaths for runtime testing

This item will be conflict-resolved whereas RuntimeCopyLocalItems was not.

Also fix the case where a package intentionally provides no assets (yet installs)

* Don't include runtime dependencies when reference was excluded by conflict resolution

* Fix import conventions in outer build

* Workaround conflict resolution on .NETStandard runtime

.NETStandard doesn't conflict resolve runtime assets.
Workaround by feeding .NETStandard references as runtime for purposes of conflict resolution.

* Remove license header from non-shipping files

* Remove obsolete Project attributes

* Remove xml header and remove DefaultTargets


Commit migrated from https://github.com/dotnet/corefx/commit/c390ce7df50252e11f5d322276e9d19e046d1332

Commit migrated from https://github.com/dotnet/corefx/commit/c0c16a32abff07944f38771f64c6ada398adadc9

Exclude these from testing


Commit migrated from https://github.com/dotnet/corefx/commit/f0279bfa494e5888455a39f31a99cb2ec4b86ea9

Package testing can now use MSBuild to restore and evaluate the contents
of packages.  Define targets for evaluating closure and examining
duplicate types / assemblies.

I've defined conventions for placing targets files with framework
specific configuration, package specific configuration, and framework +
package specific configuration.

I've included the minimum needed to make these run.  For these
exceptions I will file issues.


Commit migrated from https://github.com/dotnet/corefx/commit/6b2a3fe8ec3ab26627f5e8eb7abaace03eafa185