Add explicit visibility on the self package in EffectiveVisibilityRules.

Modules in a package is implicitly visible within the same package.
When making effective visibility rules which work from another location
that visibility needs to be mentioned explicitly. In practice this
allows prebuilts to remain visible to other modules in the source
package when the prebuilts are preferred.

Test: m nothing
Test: m conscrypt-module-sdk, check that the snapshot Soong modules give
  visibility to //external/conscrypt
Bug: 151303681
Change-Id: I404f6e5ca7021974a8c7be5e4d6b4982c050b8a5