Lint: lint and fix src/{os,reflect} (#4228)

* lint: expand to src/{os,reflect}, fix or suppress what it found

* internal/tools/tools.go: the tools idiom requires a build tag guard to avoid go test complaints

src/os/file.go

@@ -310,6 +310,7 @@ func (e *LinkError) Unwrap() error {
 	return e.Err
 }
 
+// OpenFile flag values.
 const (
 	O_RDONLY int = syscall.O_RDONLY
 	O_WRONLY int = syscall.O_WRONLY

src/os/path_unix.go

@@ -7,8 +7,8 @@
 package os
 
 const (
-	PathSeparator     = '/' // OS-specific path separator
-	PathListSeparator = ':' // OS-specific path list separator
+	PathSeparator     = '/' // PathSeparator is the OS-specific path separator
+	PathListSeparator = ':' // PathListSeparator is the OS-specific path list separator
 )
 
 // IsPathSeparator reports whether c is a directory separator character.

src/os/path_windows.go

@@ -5,8 +5,8 @@
 package os
 
 const (
-	PathSeparator     = '\\' // OS-specific path separator
-	PathListSeparator = ';'  // OS-specific path list separator
+	PathSeparator     = '\\' // PathSeparator is the OS-specific path separator
+	PathListSeparator = ';'  // PathListSeparator is the OS-specific path list separator
 )
 
 // IsPathSeparator reports whether c is a directory separator character.