Use an external package for retry logic in source-fetcher

[haltman-at]

Issue

The source-fetcher package currently implements its own retry logic. It would be better to rely on an external package for this. @gnidan recently started using the axios-retry package in his compiler supplier refactor; we could use that.

Environment

• Truffle version (truffle version): 5.4.3

[haltman-at]

Hm so unfortunately, axios-retry won't work for interfacing with Etherscan. (We can use it for Sourcify! But not Etherscan.) The problem is that axios-retry doesn't allow retrying on a 200 OK response, but Etherscan often indicates errors by sending a 200 OK and then indicating the error in the body. So, that won't work here. (I've filed an issue with them, FWIW.)