[Build] Clarify BuildPlan.Error.missingLinuxMain (#1920)

* [Build] Clarify `BuildPlan.Error.missingLinuxMain`

Make it clear that LinuxMain.swift must be at the top level.
[SR-8646] BuildPlan.Error.missingLinuxMain description rephrasing

* [Build] Reclarify `BuildPlan.Error.missingLinuxMain`

Make it clear that LinuxMain.swift must be in the Tests directory, rather than just an arbitrary "top level".
[SR-8646] BuildPlan.Error.missingLinuxMain description rephrasing

Sources/Build/BuildPlan.swift

@@ -788,7 +788,7 @@ public class BuildPlan {
         public var description: String {
             switch self {
             case .missingLinuxMain:
-                return "missing LinuxMain.swift file"
+                return "missing LinuxMain.swift file in the Tests directory"
             case .noBuildableTarget:
                 return "the package does not contain a buildable target"
             }