x/pkgsite: support searching for internal packages

[stamblerre]

For example: https://pkg.go.dev/search?q=internal%2Flsp should return results including https://pkg.go.dev/golang.org/x/tools/internal/lsp?tab=doc.

[myitcv]

Probably related to #36803 #36804 and #36806

[neild]

As a related issue, but in the other direction: If I look at https://pkg.go.dev/google.golang.org/protobuf/, I see both internal and external packages in the module packages list. I don't think the internal ones should be there by default, or should be in a separate section after the external ones. They distract from the public interface of the module.