Developer-facing function that recomputes MD5 checksums for all raw source
files currently present in the raw data folder and saves them to
data/pgchecksum.rda. Replaces the manual data_raw/pgchecksum.R script.
Details
Only run this when you have a fully verified, clean set of downloaded files.
The resulting pgchecksum object is bundled with the package and used by
check_pgsourcefiles() and the optional checksum verification in get_pgfile().