duplicates:

• SR-5383 Display appropriate message when trying to build system module packages

Issue Description:

I am trying to build a swift system package on Ubuntu which uses Berkeley DB. It generated following errors

<unknown>:0: error: unexpected 'commands' value (expected map)
{{ <unknown>:0: error: unable to load build file}}

My modulemap file looks like the following

module BerkeleyDB [system] {
{{ header "/usr/include/db.h"}}
{{ link "libdb-5.3"}}
{{ export *}}
{{ }}}
{{ }}

And Package.swift is as follows
// swift-tools-version:4.0
{{ // The swift-tools-version declares the minimum version of Swift required to build this package.}}

import PackageDescription

let package = Package(
{{ name: "BerkeleyDB",}}
{{ dependencies: [}}
{{ // Dependencies declare other packages that this package depends on.}}
{{ // .package(url: /* package url */, from: "1.0.0"),}}
{{ ]}}
{{ )}}