cmd/go: test 'go get' and 'go install' with an (invalid) module ending in /v1

We expect major version v1 to not have a /v1 suffix.
(Such a suffix on the package path is fine, but not on the module path.)

Fixes #26375

Change-Id: I4206ded82ede36440cedfaf39875c38b9c95dc27
Reviewed-on: https://go-review.googlesource.com/c/153823
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/testdata/mod/example.com_invalidpath_v1_v1.0.0.txt

@@ -0,0 +1,13 @@
+example.com/invalidpath/v1 v1.0.0
+written by hand
+
+-- .mod --
+module example.com/invalidpath/v1
+-- .info --
+{"Version":"v1.0.0"}
+-- go.mod --
+module example.com/invalidpath/v1
+-- version.go --
+package version
+
+const V = "v1.0.0"

src/cmd/go/testdata/script/mod_import_v1suffix.txt

@@ -0,0 +1,11 @@
+env GO111MODULE=on
+
+! go get -m example.com/invalidpath/v1
+! go install .
+
+-- go.mod --
+module example.com
+-- main.go --
+package main
+import _ "example.com/invalidpath/v1"
+func main() {}