Retrieve providers list if no submodule is found

[CasperWA]

Fixes #373

The logic is now:

1. Try to fetch providers from optimade.server.data:providers.json.
2. If not found, try to fetch providers from https://providers.optimade.org/v1/links.
3. If not found, raise.

[codecov]

Codecov Report

Merging #374 into master will decrease coverage by 0.48%.
The diff coverage is 35.00%.

@@            Coverage Diff             @@
##           master     #374      +/-   ##
==========================================
- Coverage   90.92%   90.43%   -0.49%     
==========================================
  Files          54       54              
  Lines        2335     2352      +17     
==========================================
+ Hits         2123     2127       +4     
- Misses        212      225      +13     

Flag	Coverage Δ
#unittests	90.43% <35.00%> (-0.49%)	⬇️

Impacted Files	Coverage Δ
optimade/server/routers/utils.py	91.03% <28.57%> (-6.70%)	⬇️
optimade/server/data/__init__.py	70.00% <50.00%> (-30.00%)	⬇️

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[shyamd]

I think the loading of data in optimade.server.data should be the robust part. That way we're single-sourcing where the rest of the module gets "data" from.

[shyamd]

Ideally, we eliminate get_providers or put that and similar functions into optimade.server.data

[ml-evs]

Ideally, we eliminate get_providers or put that and similar functions into optimade.server.data

We're trying to unblock a load of other PRs along the way here, but you're right, think this was the eventual intention, we should come back to it ;)