cmd/go/internal/modload: use vendorMap in findModule

The build list is very incomplete in vendor mode,
so we can't rely on it in general.
findModule may be called in modload.PackageModuleInfo, which
load.LoadImport invokes relatively early during a build.

Before this change, the accompanying test failed at 'go build
-mod=vendor' with the message:

	build diamondpoint: cannot find module for path diamondpoint

Change-Id: I5e667d8e406872be703510afeb079f6cbfdbd3c8
Reviewed-on: https://go-review.googlesource.com/c/140861
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/modload/build.go

@@ -228,6 +228,10 @@ func findModule(target, path string) module.Version {
 	if path == "." {
 		return buildList[0]
 	}
+	if cfg.BuildMod == "vendor" {
+		readVendorList()
+		return vendorMap[path]
+	}
 	for _, mod := range buildList {
 		if maybeInModule(path, mod.Path) {
 			return mod

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

@@ -67,6 +67,7 @@ module m
 
 require (
 	a v1.0.0
+	diamondroot v0.0.0
 	mysite/myname/mypkg v1.0.0
 	w v1.0.0 // indirect
 	x v1.0.0
@@ -76,6 +77,10 @@ require (
 
 replace (
 	a v1.0.0 => ./a
+	diamondleft => ./diamondleft
+	diamondpoint => ./diamondpoint
+	diamondright => ./diamondright
+	diamondroot => ./diamondroot
 	mysite/myname/mypkg v1.0.0 => ./mypkg
 	w v1.0.0 => ./w
 	x v1.0.0 => ./x
@@ -200,6 +205,10 @@ import _ "z"
 package m
 
 import _ "x/x1"
+-- importdiamond.go --
+package m
+
+import _ "diamondroot"
 -- w/go.mod --
 module w
 -- w/w.go --
@@ -228,3 +237,42 @@ package y
 module z
 -- z/z.go --
 package z
+
+-- diamondroot/go.mod --
+module diamondroot
+
+require (
+	diamondleft v0.0.0
+	diamondright v0.0.0
+)
+-- diamondroot/x.go --
+package diamondroot
+
+import (
+	_ "diamondleft"
+	_ "diamondright"
+)
+-- diamondleft/go.mod --
+module diamondleft
+
+require (
+	diamondpoint v0.0.0
+)
+-- diamondleft/x.go --
+package diamondleft
+
+import _ "diamondpoint"
+-- diamondright/go.mod --
+module diamondright
+
+require (
+	diamondpoint v0.0.0
+)
+-- diamondright/x.go --
+package diamondright
+
+import _ "diamondpoint"
+-- diamondpoint/go.mod --
+module diamondpoint
+-- diamondpoint/x.go --
+package diamondpoint