Move most code from the repo root into subdirectories

The intention is to keep to repo root clean while the list of packages
is slowly growing.

Additionally, a bunch of small (~30 LoC) files in the repo root is
merged into a single maddy.go file, for the same reason.

Most of the internal code is moved into the internal/ directory. Go
toolchain will make it impossible to import these packages from external
applications.

Some packages are renamed and moved into the pkg/ directory in the root.
According to https://github.com/golang-standards/project-layout this is
the de-facto standard to place "library code that's ok to use by
external applications" in.

To clearly define the purpose of top-level directories, README.md files
are added to each.

signal.go

@@ -7,7 +7,7 @@ import (
 	"os/signal"
 	"syscall"
 
-	"github.com/foxcpp/maddy/log"
+	"github.com/foxcpp/maddy/internal/log"
 )
 
 // handleSignals function creates and listens on OS signals channel.