Adding support for native-txm folder

[emgarten]

This change adds support for /runtimes/{rid}/native-txm/{tfm}/ Nupkgs can use this to specify a native dependency that pivots on both framework and rid.

NuGet/Home#2782

//cc @alpaix @zhili1208 @rohit21agrawal @rrelyea @ericstj @davidfowl

[ericstj]

Why did you have to use native-txm instead of just native and have it behave similar to MSBuildFiles? (eg: prefer TFM folder if it exists, but fallback to no-TFM)

[emgarten]

@ericstj that behavior has been a problem for the lib folder since all framework names must be hardcoded/known for it to work consistently.

Im open to any suggestions on the path, I'd just really like to avoid tfm/sub folder conflicts completely.

[ericstj]

Are you worried about packages that exist today that had a net45 folder which didn't expect for it to be a TFM filter? Are you worried about packages that have a 'foo' folder today and we create a 'foo' TFM sometime in the future?

[rohit21agrawal]

[davidfowl]

native-txm looks so odd.

[rrelyea]

Yes, we renamed it to "assets", then "nativeassets". Discussion is in the PR, I believe.