Remove corrupt package references in db recover

When aptly crashes it is possible to get a corrupt database with a dangling key reference.
This results in an error with 'key not found', eg:

    ERROR: unable to load package Pall example-package 1.2.3 778cf6f877bf6e2d: key not found

This change makes `db recover` fix this situation by removing the dangling references.

deb/find_dangling_test.go

@@ -0,0 +1,46 @@
+package deb_test
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/aptly-dev/aptly/database/goleveldb"
+	"github.com/aptly-dev/aptly/deb"
+)
+
+func TestFindDanglingReferences(t *testing.T) {
+	reflist := deb.NewPackageRefList()
+	reflist.Refs = [][]byte{[]byte("P existing 1.2.3"), []byte("P dangling 1.2.3")}
+
+	db, _ := goleveldb.NewOpenDB(t.TempDir())
+	packages := deb.NewPackageCollection(db)
+
+	if err := packages.Update(&deb.Package{Name: "existing", Version: "1.2.3"}); err != nil {
+		t.Fatal(err)
+	}
+
+	dangling, err := deb.FindDanglingReferences(reflist, packages)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	exp := &deb.PackageRefList{
+		Refs: [][]byte{[]byte("P dangling 1.2.3")},
+	}
+
+	compareRefs(t, exp, dangling)
+}
+
+func compareRefs(t *testing.T, exp, got *deb.PackageRefList) {
+	t.Helper()
+
+	if len(exp.Refs) != len(got.Refs) {
+		t.Fatalf("refs length mismatch: exp %d, got %d", len(exp.Refs), len(got.Refs))
+	}
+
+	for i := range exp.Refs {
+		if !bytes.Equal(exp.Refs[i], got.Refs[i]) {
+			t.Fatalf("refs do not match: exp %q, got %q", exp.Refs[i], got.Refs[i])
+		}
+	}
+}