Exclude source->prebuilt deps from visibility enforcement

When both prebuilt and source versions of a module are present in the
build an implicit dependency is added from source -> prebuilt. This
change excludes that dependency from visibility enforcement as it
provides no value and would otherwise require an sdk snapshot to
rewrite the visibility of the generated prebuilt modules which would
be complicated.

The rewriting would include:
* Detecting //visibility:public and just passing that straight through.
* Detecting //visibility:private and replacing that with the location
  of the source.
* Otherwise, adding the location of the source to the visibility
  property.

This adds a general mechanism to allow any dependency to be excluded
from visibility enforcement by simply using a tag that implements
the ExcludeFromVisibilityEnforcementTag interface.

Bug: 142940300
Test: m nothing
Change-Id: I0668ff5aa798152d17faf3aac1bb8eff8d6350c3

android/prebuilt.go

@@ -36,6 +36,9 @@ type prebuiltDependencyTag struct {
 
 var PrebuiltDepTag prebuiltDependencyTag
 
+// Mark this tag so dependencies that use it are excluded from visibility enforcement.
+func (t prebuiltDependencyTag) ExcludeFromVisibilityEnforcement() {}
+
 type PrebuiltProperties struct {
 	// When prefer is set to true the prebuilt will be used instead of any source module with
 	// a matching name.