Check module existence with ALL_MODULES subvars

Use ALL_MODULES.*.PATH to check module existence.
Use get-modules-for-2nd-arch to check the existence of second arch
variant.
Fix some typos *existant -> existent

Bug: 155869107
Test: lunch mainline_system_arm64; m
Test: lunch generic; m
Change-Id: Ic10fb34a11da0bfbfc528da7496fd929b1d77113