Feature compile multi sources in a subfolder

[prcolaco]

This PR implements the feature of compiling multiple source files to the same contract, if they are grouped inside a subfolder of the one that is passed to --path, or default ./contracts.

This means that the behaviour is the same as before if: ./contracts/contract1.cpp and ./contracts/contract2.cpp,
but the new behaviour will be present if: ./contracts/mycontract/mycontract.cpp and ./contracts/mycontract/helpers.cpp
resulting in compiling contract mycontract by linking together both mycontract.cpp and helpers.cpp source files.

Also included unit tests for both behaviours as well.

[enginterzi]

@prcolaco
I think this is a very good feature but it will fail if your .hpp files are in a different folder. If that is the case you should also add an include parameter with the folder name.

For example;
eosio-cpp -abigen -I ./include ....

[prcolaco]

@enginterzi I agree that this is needed, but I think it is a different issue, since different include directory was never supported anyway... but good idea to add. 👍