[unused_hrls] Figure out the actual path of an include file, don't guess

[elbrujohalcon]

Currently, unused_hrls might retrun some false negatives.
For instance, it may think that lib/app1/include/header.hrl is used if lib/app2/src/module.erl contains -include("header.hrl"). when, in reality, module.erl is including lib/app2/include/header.erl or lib/app2/src/header.hrl.
To prevent this situation we need to properly retrieve the absolute path of "header.hrl" in the same way that the Erlang pre-processor retrieves it.