Use consistent naming for covidcast data_sources/sources

[brookslogan]

Use the same base name for data-source-associated:

• parameters to covidcast and related functions
• elements of covidcast_epidata()
• columns of covidcast results
• etc.

Either source and sources or data_source and data_sources.

[dshemetov]

So this is tricky because:

• source overwrites a keyword in R
• sources is misleading in covidcast, because it only accepts one source
• data_source(s) are both long for covidcast_epidata (which aims to be a handy autocomplete thing)

I'm going with (1), even though it feels slightly wrong: I really don't expect anyone to need the source keyword inside the covidcast function.