main: synchronize writes to VT100-faker on Windows

We use a third-party library "colorable" to translate VT100 color
sequences into Windows console attribute-setting calls when Terraform is
running on Windows.

colorable is not concurrency-safe for multiple writes to the same console,
because it writes to the console one character at a time and so two
concurrent writers get their characters interleaved, creating unreadable
garble.

Here we wrap around it a synchronization mechanism to ensure that there
can be only one Write call outstanding across both stderr and stdout,
mimicking the usual behavior we expect (when stderr/stdout are a normal
file handle) of each Write being completed atomically.

main.go

@@ -258,6 +258,15 @@ func copyOutput(r io.Reader, doneCh chan<- struct{}) {
 	if runtime.GOOS == "windows" {
 		stdout = colorable.NewColorableStdout()
 		stderr = colorable.NewColorableStderr()
+
+		// colorable is not concurrency-safe when stdout and stderr are the
+		// same console, so we need to add some synchronization to ensure that
+		// we can't be concurrently writing to both stderr and stdout at
+		// once, or else we get intermingled writes that create gibberish
+		// in the console.
+		wrapped := synchronizedWriters(stdout, stderr)
+		stdout = wrapped[0]
+		stderr = wrapped[1]
 	}
 
 	var wg sync.WaitGroup