envsetup.sh: fix adb tab completion

The adb module has been moved from system/core to packages/modules/adb.
Update the path to the adb tab completion script accordingly.

Also teach envsetup.sh to complain if completion files are not found.

Test: manual: adb tab completion works
Test: source build/envsetup.sh && declare -F _adb
Change-Id: I7d6fda3813b95e64de7b1953675f66659e750c31