all: change module name to filippo.io/age

No need to tie ourselves to GitHub.

The redirect is not set up yet, but as long as there is a replace in the
go.mod the tool can't be installed with "go get" anyway.

Not using age-tool.com because A) I don't actually like the domain and
B) it should be about the spec not the specific implementation.

cmd/age-keygen/keygen.go

@@ -14,7 +14,7 @@ import (
 	"os"
 	"time"
 
-	"github.com/FiloSottile/age/internal/age"
+	"filippo.io/age/internal/age"
 )
 
 func main() {

cmd/age/age.go

@@ -14,7 +14,7 @@ import (
 	"log"
 	"os"
 
-	"github.com/FiloSottile/age/internal/age"
+	"filippo.io/age/internal/age"
 	"golang.org/x/crypto/ssh/terminal"
 )
 

cmd/age/encrypted_keys.go

@@ -12,8 +12,8 @@ import (
 	"fmt"
 	"os"
 
-	"github.com/FiloSottile/age/internal/age"
-	"github.com/FiloSottile/age/internal/format"
+	"filippo.io/age/internal/age"
+	"filippo.io/age/internal/format"
 	"golang.org/x/crypto/ssh"
 	"golang.org/x/crypto/ssh/terminal"
 )

cmd/age/parse.go

@@ -15,7 +15,7 @@ import (
 	"os"
 	"strings"
 
-	"github.com/FiloSottile/age/internal/age"
+	"filippo.io/age/internal/age"
 	"golang.org/x/crypto/ssh"
 )