pkgsite: doesn't handle non-ascii package name, regression compared to godoc #43036
Description
Discovered while investigating #43035 :
pkg.go.dev fails at loading a non-ascii package name https://pkg.go.dev/github.com/maruel/panicparse/v2/cmd/panic/internal/%C3%B9tf8
but godoc succeeds: https://godoc.org/github.com/maruel/panicparse/cmd/panic/internal/%c3%b9tf8
Feels like a regression to me. @julieqiu