[Validator] Extract source subdivisions from /covidcast/meta instead of hard-coding

[krivard]

Validator currently hard-codes the mapping from sources to source subdivisions:

covidcast-indicators/_delphi_utils_python/delphi_utils/validator/datafetcher.py

Lines 114 to 119 in 716f733

	source_signal_mappings = {
	'chng': ['chng-cli', 'chng-covid'],
	'indicator-combination': ['indicator-combination-cases-deaths'],
	'quidel': ['quidel-covid-ag'],
	'safegraph': ['safegraph-weekly']
	}

This means whenever we change the mapping in the spreadsheet (eg for cmu-delphi/delphi-epidata#800), we would need to remember the extra step of updating this hardcoded copy in validator. That is not ideal.

Is this mapping exposed as part of /covidcast/meta? If so, we should have validator look it up there instead. If not, we should add an endpoint to delphi-epidata /covidcast/* to surface it.

[krivard]

mata with no arguments gives you db_source and then source for all the member signals, which is exactly this mapping.