Compile std output indicates the same contract several times

[armaniferrante]

When building some of the runtime test contracts, oasis-compile outputs the following

Building contracts for Oasis
   Compiling basic_wasm...
   Compiling rust-logistic...
   Compiling storage...
   Compiling deployed...
   Compiling existing...
   Compiling Builtins.sol...
   Compiling Builtins.sol...
   Compiling Builtins.sol...
   Compiling Counter.sol...
...

Note that Compiling Builtins.sol... is output 3 times even though there is one contract. Also, note that only one contract artifact is actually created, which is correct.

I suspect this because the Builtins.sol file actually has 1 contract with two libraries.

Acceptance criteria: correctly compile the artifacts while output Compiling Builtins.sol... once.