disallow github and bitbucket hosts

These hosts have special usage in module source strings, and can't be
valid registry hosts.

registry/regsrc/module.go

@@ -32,6 +32,13 @@ var (
 	moduleSourceRe = regexp.MustCompile(
 		fmt.Sprintf("^(%s)\\/(%s)\\/(%s)(?:\\/\\/(.*))?$",
 			nameSubRe, nameSubRe, providerSubRe))
+
+	// disallowed is a set of hostnames that have special usage in modules and
+	// can't be registry hosts
+	disallowed = map[string]bool{
+		"github.com":    true,
+		"bitbucket.org": true,
+	}
 )
 
 // Module describes a Terraform Registry Module source.
@@ -77,9 +84,11 @@ func NewModule(host, namespace, name, provider, submodule string) *Module {
 func ParseModuleSource(source string) (*Module, error) {
 	// See if there is a friendly host prefix.
 	host, rest := ParseFriendlyHost(source)
-	if host != nil && !host.Valid() {
+	if host != nil {
+		if !host.Valid() || disallowed[host.Display()] {
 			return nil, ErrInvalidModuleSource
 		}
+	}
 
 	matches := moduleSourceRe.FindStringSubmatch(rest)
 	if len(matches) < 4 {

registry/regsrc/module_test.go

@@ -96,6 +96,16 @@ func TestModule(t *testing.T) {
 			source:  "foo.com/var/baz?otherthing",
 			wantErr: true,
 		},
+		{
+			name:    "disallow github",
+			source:  "github.com/HashiCorp/Consul/aws",
+			wantErr: true,
+		},
+		{
+			name:    "disallow bitbucket",
+			source:  "bitbucket.org/HashiCorp/Consul/aws",
+			wantErr: true,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {