Issue Description:

If I follow the [pattern](https://swift.org/package-manager/#resolving-transitive-dependencies) used by the Dealer example, but I fail to call a source file `main.swift`, I get

"'Example' /Users/dave/src/Example: error: executable product 'Example' should have exactly one executable target"

The `Package.swift` file is just fine; it's just the fact that I called my file `Example.swift` instead of `main.swift`.