astro/xtide: Fix missing distfile and unbreak
- Make the tarball for the harmonics file fetchable again as it has been updated to a newer version a while ago.
- Also update the WWW field to spare users from redirects while I'm here.
PR: 239666
Submitted by: Walter Schwarzenfeld <w.schwarzenfeld@utanet.at> (based on)
MFH: 2019Q3