[Build] Clarify BuildPlan.Error.missingLinuxMain (swiftlang#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

Author: matthewseaman

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"
  }