Removing RID specific assets from compile

[emgarten]

This change removes RID specific assets in the runtimes/ folder from the compile section of the assets file.

When referencing a package that contained only runtimes/ assets it was possible to fallback to using these assets under compile.

Previous behavior:

1. Select compile assets from ref/
2. If no ref/ assets exist try lib/ and runtimes/ assets.

Fixed behavior:

1. Select compile assets from ref/
2. If no ref/ assets exist try lib/ assets.

Risk

Users depending directly on runtime only packages and not the ref package could see build breaks when the compile assets are missing. In this scenario it seems like users should just fix their references.

Fixes NuGet/Home#4207

//cc @ericstj @alpaix @rrelyea

[emgarten]

//cc @joelverhagen