Merge changes Ia968e06b,I60192285,I0e857a64,Ia8880fe5,If7a92ca0, ... into main

* changes:
  Add explicitlyImpl to shared library dependency tags
  Don't enforce visiblity on platformBootclasspathImplLibDepTag
  Don't merge apex variations with different UsePlatformApis
  Use unique apex variations for bootclasspath fragments
  Correctly compute externalDep for dependencies of override_apex modules
  Remove extraneous variation name from OverrideApex