Downloading extra distfiles from a different URL

[Mayuresh <mayuresh%acm.org@localhost>]

I have created wip/ThirdParty-ASL as a part of building openlane2 on
NetBSD.

Please see TODO in wip/ThirdParty-ASL. I have put some ad-hoc code there
to download and extract the distfile.

The FreeBSD port does this more appropriately by making it a distfile.

In pkgsrc how does one use different MASTER_SITES for different distfiles?
(If I try adding a MASTER_SITE it says, this being a github project the
site must be github.com.)

-- 
Mayuresh