x/pkgsite: Go 1.18 packages with generics are not listed

[gmlewis]

(I know that Go 1.18 hasn't been released yet, but thought it might be helpful to get a jump on this bug before its release.)

What is the URL of the page with the issue?

https://pkg.go.dev/github.com/gmlewis/advent-of-code-2021@v0.0.6

What is your user agent?

Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.45 Safari/537.36

Screenshot

What did you do?

I requested that https://pkg.go.dev/github.com/gmlewis/advent-of-code-2021@v0.0.6 be generated.

What did you expect to see?

I expected to also see the packages: enum, maps, and stream (in addition to the correctly-displayed must, strfn, and test packages).

What did you see instead?

Only the packages without Go 1.18 generics are displayed: must, strfn, and test.

[fzipp]

Duplicate of #48264