Consolidate runtime library deduplication

[MarijnS95]

We merged a first stab at deduplicating runtime libraries in rust-mobile/ndk#333, to prevent cargo-apk from complaining that an identically-named library (or file in general) has been added twice (when two or more recursively added libraries depend on the same thing).

There may however be a few things to improve:

• Don't (recursively) search for libraries when they've already been found and are pending inclusion;
• When the source path (and checksum) for a duplicate library differs (i.e. something found in the search path versus an identically named file specified in runtime_libs), warn the user as this might not be intended.