crypto/secp256k1: add workaround for go mod vendor (#21735)

Go won't vendor C files if there are no Go files present in the directory.
Workaround is to add dummy Go files.

Fixes: #20232

crypto/secp256k1/dummy.go

+// +build dummy
+
+// This file is part of a workaround for `go mod vendor` which won't vendor
+// C files if there's no Go file in the same directory.
+// This would prevent the crypto/secp256k1/libsecp256k1/include/secp256k1.h file to be vendored.
+//
+// This Go file imports the c directory where there is another dummy.go file which
+// is the second part of this workaround.
+//
+// These two files combined make it so `go mod vendor` behaves correctly.
+//
+// See this issue for reference: https://github.com/golang/go/issues/26366
+
+package secp256k1
+
+import (
+	_ "github.com/ethereum/go-ethereum/crypto/secp256k1/libsecp256k1/include"
+	_ "github.com/ethereum/go-ethereum/crypto/secp256k1/libsecp256k1/src"
+	_ "github.com/ethereum/go-ethereum/crypto/secp256k1/libsecp256k1/src/modules/recovery"
+)

crypto/secp256k1/libsecp256k1/contrib/dummy.go

+// +build dummy
+
+// Package c contains only a C file.
+//
+// This Go file is part of a workaround for `go mod vendor`.
+// Please see the file crypto/secp256k1/dummy.go for more information.
+package contrib

crypto/secp256k1/libsecp256k1/dummy.go

+// +build dummy
+
+// Package c contains only a C file.
+//
+// This Go file is part of a workaround for `go mod vendor`.
+// Please see the file crypto/secp256k1/dummy.go for more information.
+package libsecp256k1

crypto/secp256k1/libsecp256k1/include/dummy.go

+// +build dummy
+
+// Package c contains only a C file.
+//
+// This Go file is part of a workaround for `go mod vendor`.
+// Please see the file crypto/secp256k1/dummy.go for more information.
+package include

crypto/secp256k1/libsecp256k1/src/dummy.go

+// +build dummy
+
+// Package c contains only a C file.
+//
+// This Go file is part of a workaround for `go mod vendor`.
+// Please see the file crypto/secp256k1/dummy.go for more information.
+package src

crypto/secp256k1/libsecp256k1/src/modules/dummy.go

+// +build dummy
+
+// Package c contains only a C file.
+//
+// This Go file is part of a workaround for `go mod vendor`.
+// Please see the file crypto/secp256k1/dummy.go for more information.
+package module

crypto/secp256k1/libsecp256k1/src/modules/ecdh/dummy.go

+// +build dummy
+
+// Package c contains only a C file.
+//
+// This Go file is part of a workaround for `go mod vendor`.
+// Please see the file crypto/secp256k1/dummy.go for more information.
+package ecdh

crypto/secp256k1/libsecp256k1/src/modules/recovery/dummy.go

+// +build dummy
+
+// Package c contains only a C file.
+//
+// This Go file is part of a workaround for `go mod vendor`.
+// Please see the file crypto/secp256k1/dummy.go for more information.
+package recovery