Put file of verified contract first in the list of sources

[fvictorio]

When a contract is verified, we build a solc input that is sent to Etherscan. This input includes a list of sources, and Etherscan will show these sources in that order. Ideally the first source should correspond to the file of the contract that is being verified.

This is relatively easy to do, because we already have the file of the verified contract (we use it to compute the minimal input to send).

There are other improvements we could make, but that are not part of this issue. I'm mentioning here for completeness:

• Sort the sources in reverted topological order, starting from the one of the verified contract, so that each file shown was imported by a file shown previously.
• Provide some mechanism (like an overridable sub-task) to configure the way the sources are sorted.