commandline: Refactor and streamline special syntaxes

All those special syntaxes only trigger if args.length >= 2,
so first we can nest them in a new scope, which also makes
it visually more distinct.

Additionally, the third and second cases were actually inverted:
if the third case was to pass, it means the second case couldn't
have passed (because they check present/absence of an extension,
respectively), so we can swap them and use an else-if.

The result is arguably much more readable, and highlight the fact
that if the first case (stdin) is used, the second case will.

source/dub/commandline.d

@@ -403,28 +403,32 @@ int runDubCommandLine(string[] args)
 	auto handler = CommandLineHandler(getCommands());
 	auto commandNames = handler.commandNames();
 
-	// special stdin syntax
-	if (args.length >= 2 && args[1] == "-")
+	// Special syntaxes need to be handled before regular argument parsing
+	if (args.length >= 2)
 	{
-		auto path = getTempFile("app", ".d");
-		stdin.byChunk(4096).joiner.toFile(path.toNativeString());
-		args = args[0] ~ [path.toNativeString()] ~ args[2..$];
-	}
-
-	// Shebang syntax support for files without .d extension
-	if (args.length >= 2 && !args[1].endsWith(".d") && !args[1].startsWith("-") && !commandNames.canFind(args[1])) {
-		if (exists(args[1])) {
+		// Read input source code from stdin
+		if (args[1] == "-")
+		{
 			auto path = getTempFile("app", ".d");
-			copy(args[1], path.toNativeString());
-			args[1] = path.toNativeString();
-		} else if (exists(args[1].setExtension(".d"))) {
-			args[1] = args[1].setExtension(".d");
+			stdin.byChunk(4096).joiner.toFile(path.toNativeString());
+			args = args[0] ~ [path.toNativeString()] ~ args[2..$];
 		}
-	}
 
-	// special single-file package shebang syntax
-	if (args.length >= 2 && args[1].endsWith(".d")) {
-		args = args[0] ~ ["run", "-q", "--temp-build", "--single", args[1], "--"] ~ args[2 ..$];
+		// special single-file package shebang syntax
+		if (args[1].endsWith(".d")) {
+			args = args[0] ~ ["run", "-q", "--temp-build", "--single", args[1], "--"] ~ args[2 ..$];
+		}
+
+		// Shebang syntax support for files without .d extension
+		else if (!commandNames.canFind(args[1])) {
+			if (exists(args[1])) {
+				auto path = getTempFile("app", ".d");
+				copy(args[1], path.toNativeString());
+				args[1] = path.toNativeString();
+			} else if (exists(args[1].setExtension(".d"))) {
+				args[1] = args[1].setExtension(".d");
+			}
+		}
 	}
 
 	auto common_args = new CommandArgs(args[1..$]);