Move the Go module to sneak.berlin/go/keyfunc (closes #15)

REPO_POLICIES.md sets the Go module root to sneak.berlin/go/<name>.
go.mod, every import of the old path and the -X path in the Makefile
LDFLAGS now use sneak.berlin/go/keyfunc; make fmt moved those imports
to their new place in the sort order. The old path keeps no alias. The
README gives the go install line next to the build from a clone and
drops this issue from its TODO list, which now names the open 1.0
issues.

Model: opus-5-5

internal/mnemonic/mnemonic_test.go

@@ -4,8 +4,8 @@ import (
 	"strings"
 	"testing"
 
-	"git.eeqj.de/sneak/keyfunc/internal/mnemonic"
 	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/keyfunc/internal/mnemonic"
 )
 
 // Two more mnemonics that pass the checksum, so a test can tell which