Better `.checksums` file

[Xercesblu3]

• Store checksums in a single file checksums in the www directory - There is a standard format for this
• If files to be downloaded are already downloaded (except checksums, ALWAYS download the checksums) - Do the If Modified Since check - If not modified fetch the checksum file anyway and compare from disk
• If NUMPTY set (in Deno.env) and fetch fails (eg no Internet), use checksum from the checksums file - so yes, we only use the checksums file if the user forces it