Make package size and checksum mandatory in setup.ini

Make package size and checksum mandatory in setup.ini

The setup.ini grammar is written such that size and checksum are optional in
install: lines, and checksum is optional in source: lines.  Presumably this
had some historical use.

Change to make package size and checksum mandatory.

Future work: We keep the asymmetry between install: and source: where
buildPackageSource() sets both pathname and size, but buildPackageInstall()
only sets pathname, relying on a separate buildInstallSize() to set the size
later for the moment, as fixing that ripples through to other place.