support /index

[drewwells]

There's a desire for people to run private module indexes and sumdb without relying on google's pkgsite. However, similar projects like athens do not support the index feed needed to maintain these. It would be great if goproxy added support for /index and /latest. These are used by pkgsite to find new modules and regularly update the pkgsite database.

Ref:
https://sum.golang.org/latest
https://index.golang.org/index