CommandHelpers::isGenericArgument is currently inline in commands.h.

Moving its definition to commands.cpp reveals that it was being called by other libraries that did not and could not declare the db/commands library in their LIBDEPS. Adding it to their LIBDEPS creates a cycle. We shouldn't tolerate such hidden library cycles.

isGenericArgument doesn't actually depend on anything. It's just a lookup in a whitelist of strings. It's reasonable to just make a library to collect these classification predicates that identify passthrough arguments and generic arguments. This library is lower level than the dispatching, properties, parsing, and behaviors of specific executable Command that are the responsibility of the more complex db/command library.