New crate structure: fetcher-core and (maybe) fetcher-builder

[nachog00]

After thorough discussion, we realized that the current structure is ill suited for a couple reasons:

• Setting up tests for fetcher's build script isn't trivial. Less so have it be picked up by some coverage tool.
• It isn't completely clear yet whether having all the fetched resources in fetcher's OUT_DIR is a viable idea:
  • It is vulnerable to cargo clean.
  • It is (possibly?) prone to unnecessary re fetches on new build hashes.

The initial proposed solution consists of separating the current fetcher crate into two new ones:

• fetcher-core: It implements all the actual fetching logic and defines tests on it
• fetcher-builder: It simply uses fetcher-core as a build-dep and implements a build script. It's intended use is as a dev-dep of consumers. This would make it not compile (nor fetch) on simple builds from its consumer.

Whether we want/need -builder or not isn't clear yet, as it was pointed out that -core would already implement most of the logic needed for a consumer to use it directly, as it has to take in a directory as input, where it will store the fetched resources.

We agreed then on start by building -core with emphasis on making it compatible with whatever approach afterwards:

with or without -builder.