cmd/go/internal/testdata: change list_std_vendor to use local modproxy

Also update get_panic_issue75251 to reflect that "golang.org/x/net" now
exists in the proxy. The new message is closer to the real world
behavior of the go command.

Change-Id: Icb916f7eb7628bf5340c15275f4d08086a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/752940
Auto-Submit: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>

src/cmd/go/testdata/mod/golang.org_x_net_v0.1.0.txt

@@ -0,0 +1,10 @@
+written by hand - just enough for list_std_vendor.txt
+
+-- .mod --
+module golang.org/x/net
+-- .info --
+{"Version":"v0.1.0","Name":"","Short":"","Time":"2026-03-08T00:00:00Z"}
+-- go.mod --
+module golang.org/x/net
+-- dns/dnsmessage/dnsmessage.go --
+package dnsmessage

src/cmd/go/testdata/script/get_panic_issue75251.txt

@@ -5,7 +5,7 @@ go mod init m
 
 ! go get golang.org/x/net/http/httpguts<74>v0.43.0 # contains 0xff byte
 ! stderr panic
-stderr 'malformed module path'
+stderr 'module golang.org/x/net@upgrade found.*but does not contain package golang.org/x/net/http/httpguts.v0.43.0'
 
 ! go get golang.org/x/net/http/httpguts<74>@v0.43.0 # contains 0xff byte
 ! stderr panic

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

@@ -2,6 +2,8 @@
 # dependencies regardless of whether they are listed from within or outside
 # GOROOT/src.
 
+env GOROOT=$WORK/goroot
+
 # Control case: net, viewed from outside the 'std' module,
 # should depend on vendor/golang.org/… instead of golang.org/….
 
@@ -25,9 +27,30 @@ cmp stdout $WORK/net-deps.txt
 # However, 'go mod' and 'go get' subcommands should report the original module
 # dependencies, not the vendored packages.
 
-[!net:golang.org] stop
-
-env GOPROXY=
 env GOWORK=off
 go mod why -m golang.org/x/net
 stdout '^# golang.org/x/net\nnet\ngolang.org/x/net'
+
+-- $WORK/goroot/src/go.mod --
+module std
+
+go 1.26
+
+require golang.org/x/net v0.1.0
+-- $WORK/goroot/src/go.sum --
+golang.org/x/net v0.1.0 h1:iUcD2VNMwNi3BZMUg2qIGo4aAass7E0R4eQNGK3hvc0=
+golang.org/x/net v0.1.0/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
+-- $WORK/goroot/src/vendor/modules.txt --
+# golang.org/x/net v0.1.0
+## explicit
+golang.org/x/net/dns/dnsmessage
+-- $WORK/goroot/src/vendor/modules.txt --
+# golang.org/x/net v0.1.0
+## explicit
+golang.org/x/net/dns/dnsmessage
+-- $WORK/goroot/src/vendor/golang.org/x/net/dns/dnsmessage/dnsmessage.go --
+package dnsmessage
+-- $WORK/goroot/src/net/net.go --
+package net
+
+import _ "golang.org/x/net/dns/dnsmessage"