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

envsetup.sh

@@ -395,7 +395,7 @@ function addcompletions()
     fi
 
     local completion_files=(
-      system/core/adb/adb.bash
+      packages/modules/adb/adb.bash
       system/core/fastboot/fastboot.bash
       tools/asuite/asuite.sh
     )
@@ -404,7 +404,9 @@ function addcompletions()
     # ENVSETUP_NO_COMPLETION=adb # -> disable adb completion
     # ENVSETUP_NO_COMPLETION=adb:bit # -> disable adb and bit completion
     for f in ${completion_files[*]}; do
-        if [ -f "$f" ] && should_add_completion "$f"; then
+        if [ ! -f "$f" ]; then
+          echo "Warning: completion file $f not found"
+        elif should_add_completion "$f"; then
             . $f
         fi
     done