Depend on //lib/mtl rather than a subtarget (#3821)
Depend on //lib/mtl rather than a subtarget (#3821)

This will make it easier to change mtl into a shared library.
mikejurka/engine,Hixie/sky_engine,Hixie/sky_engine,tvolkert/engine,mikejurka/engine,mikejurka/engine,jason-simmons/flutter_engine,flutter/engine,krisgiesing/sky_engine,flutter/engine,jamesr/sky_engine,jason-simmons/sky_engine,aam/engine,mikejurka/engine,krisgiesing/sky_engine,chinmaygarde/flutter_engine,chinmaygarde/flutter_engine,chinmaygarde/flutter_engine,devoncarew/engine,jamesr/flutter_engine,cdotstout/sky_engine,krisgiesing/sky_engine,chinmaygarde/flutter_engine,krisgiesing/sky_engine,tvolkert/engine,aam/engine,rmacnak-google/engine,mikejurka/engine,devoncarew/engine,devoncarew/sky_engine,Hixie/sky_engine,jason-simmons/sky_engine,cdotstout/sky_engine,jason-simmons/flutter_engine,devoncarew/sky_engine,jason-simmons/flutter_engine,flutter/engine,krisgiesing/sky_engine,flutter/engine,jason-simmons/flutter_engine,Hixie/sky_engine,chinmaygarde/sky_engine,devoncarew/sky_engine,jamesr/sky_engine,jamesr/flutter_engine,chinmaygarde/sky_engine,jamesr/flutter_engine,Hixie/sky_engine,jamesr/flutter_engine,cdotstout/sky_engine,chinmaygarde/flutter_engine,chinmaygarde/sky_engine,chinmaygarde/sky_engine,jason-simmons/sky_engine,aam/engine,aam/engine,flutter/engine,devoncarew/sky_engine,jamesr/sky_engine,chinmaygarde/flutter_engine,chinmaygarde/sky_engine,cdotstout/sky_engine,jamesr/sky_engine,jamesr/sky_engine,jamesr/sky_engine,jason-simmons/flutter_engine,devoncarew/engine,Hixie/sky_engine,krisgiesing/sky_engine,devoncarew/engine,devoncarew/engine,chinmaygarde/sky_engine,mikejurka/engine,tvolkert/engine,tvolkert/engine,rmacnak-google/engine,mikejurka/engine,jason-simmons/flutter_engine,devoncarew/engine,rmacnak-google/engine,aam/engine,cdotstout/sky_engine,tvolkert/engine,jason-simmons/sky_engine,chinmaygarde/flutter_engine,jason-simmons/flutter_engine,aam/engine,rmacnak-google/engine,Hixie/sky_engine,jamesr/flutter_engine,chinmaygarde/sky_engine,jason-simmons/sky_engine,aam/engine,rmacnak-google/engine,mikejurka/engine,rmacnak-google/engine,tvolkert/engine,jason-simmons/sky_engine,mikejurka/engine,devoncarew/sky_engine,flutter/engine,jason-simmons/flutter_engine,cdotstout/sky_engine,jamesr/flutter_engine,cdotstout/sky_engine,jamesr/flutter_engine,flutter/engine,jamesr/flutter_engine,aam/engine,flutter/engine,rmacnak-google/engine,jamesr/sky_engine,tvolkert/engine,jason-simmons/sky_engine,jamesr/flutter_engine,devoncarew/sky_engine,Hixie/sky_engine,devoncarew/engine,krisgiesing/sky_engine,devoncarew/sky_engine