Do not allow duplicate deapexer dependencies.

Without these errors, the last encountered deapexer would silently be
used. However with PRODUCT_INSTALL_APEXES there will only be deapexer
for installable APEXes, and we can assume only a single installable
APEX exists for each APEX variant, so make it an error if it isn't the
case.

Corresponding to http://ag/15156931.

Test: m nothing SOONG_CONFIG_art_module_source_build=true
Test: m nothing SOONG_CONFIG_art_module_source_build=false
Test: m nothing SOONG_CONFIG_art_module_source_build=false
  with installable:true for the com.android.art.debug prebuilt APEX -
  check that it fails with an "ambiguous duplicate deapexer" error
Bug: 192006406
Bug: 192542393
Change-Id: I44566fd26b12f82a8a67fe4a69e56303460756d0