cmd/go: search the target for patterns when -mod=vendor is set

This fixes the root cause of the bootstrap failure reported in
https://groups.google.com/d/msg/golang-dev/xcVJDj5GJ84/U0pVnUOnBwAJ.

Updates #30228

Change-Id: I9ce0898922a5aac1f61ceff30319cf88031676c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/167079
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modload/search.go

@@ -105,7 +105,14 @@ func matchPackages(pattern string, tags map[string]bool, useStd bool, modules []
 	}
 
 	if cfg.BuildMod == "vendor" {
-		walkPkgs(filepath.Join(ModRoot(), "vendor"), "", false)
+		if HasModRoot() {
+			modPrefix := Target.Path
+			if Target.Path == "std" {
+				modPrefix = ""
+			}
+			walkPkgs(ModRoot(), modPrefix, false)
+			walkPkgs(filepath.Join(ModRoot(), "vendor"), "", false)
+		}
 		return pkgs
 	}
 

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

@@ -25,6 +25,10 @@ env GOPROXY=off
 ! go list ...
 go list -mod=vendor ...
 
+# However, it should still list packages in the main module.
+go list -mod=vendor m/...
+stdout m
+
 -- go.mod --
 module m