discover DESCRIPTIONS without a populated library

[aronatkins]

Some ideas from @kevinushey:

1. Use crandb, e.g. https://crandb.r-pkg.org/dplyr/1.0.0. The JSON might need to be transformed into a more DESCRIPTION-like shape before it is shoved into the manifest. We might have whitespace differences, but not in a way that matters for package hashing.
2. Extract the DESCRIPTION files from the repos beneath https://github.com/cran.

Neither of these suggestions help with packages containing Git references. We might be better off understanding what information is needed by rsconnect + (Connect+shinyapps.io) and make sure that is available in the renv.lock.