x/pkgsite: visiting pkg.go.dev/golang.org/dl module root gives 400

[dmitshur]

Visiting https://pkg.go.dev/golang.org/dl shows 400 Bad Request:

Individual packages inside, such as https://pkg.go.dev/golang.org/dl/go1.15.6, work okay. But those pages include a link to the module root, which is broken as a result of this issue:

The golang.org/dl module is somewhat unusual in that it's the only module that begins with golang.org but doesn't have a golang.org/x prefix.

This problem does not reproduce at godoc.org: https://godoc.org/golang.org/dl#pkg-subdirectories.

CC @jamalc.

[gopherbot]

Change https://golang.org/cl/276232 mentions this issue: internal/frontend: add support for golang.org/dl