internal: remove gopkg.in from three-element vcs hosts

Fixes golang/go#49762

Change-Id: Ia52b3e8fe483057c8363f3551a4f631005667eaa
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/366578
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>

internal/frontend/section.go

diff --git a/internal/frontend/section.go b/internal/frontend/section.go
index 1986398..0a24fde 100644
--- a/internal/frontend/section.go
+++ b/internal/frontend/section.go
@@ -124,6 +124,8 @@
 	switch {
 	case internal.VCSHostWithThreeElementRepoName(parts[0]):
 		n = 1
+	case parts[0] == "gopkg.in":
+		n = 1
 	case parts[0] == "hub.jazz.net":
 		n = 2
 	default:

internal/frontend/urlinfo_test.go

diff --git a/internal/frontend/urlinfo_test.go b/internal/frontend/urlinfo_test.go
index 3722c14..7f66286 100644
--- a/internal/frontend/urlinfo_test.go
+++ b/internal/frontend/urlinfo_test.go
@@ -197,6 +197,7 @@
 		{"golang.org/x", false},
 		{"golang.org/x/tools", true},
 		{"golang.org/x/tools/go/packages", true},
+		{"gopkg.in/yaml.v2", true},
 	}
 	for _, test := range tests {
 		got := isValidPath(test.path)

internal/paths.go

diff --git a/internal/paths.go b/internal/paths.go
index 89d5059..89fd53b 100644
--- a/internal/paths.go
+++ b/internal/paths.go
@@ -20,7 +20,6 @@
 		"gitea.com",
 		"gitee.com",
 		"gitlab.com",
-		"gopkg.in",
 		"hg.sr.ht",
 		"bitbucket.org",
 		"github.com",