Rework mirror update (download packages) implementation

`PackageDownloadTask` is just a reference to file now. Whole process
was rewritten to follow pattern: download to temp location inside the pool,
verify/update checksums, import into pool as final step.

This removes a lot of edge cases when aptly internal state might be broken
if updating from rogue mirror.

Also this changes whole memory model: package list/files are kept in memory
now during the duration of `mirror update` command and saved to disk
only in the end.

deb/package_test.go

@@ -392,35 +392,25 @@ func (s *PackageSuite) TestFilepathList(c *C) {
 }
 
 func (s *PackageSuite) TestDownloadList(c *C) {
-	c.Fail()
-	/*
-		packagePool := files.NewPackagePool(c.MkDir())
-		p := NewPackageFromControlFile(s.stanza)
-		p.Files()[0].Checksums.Size = 5
-		poolPath, _ := packagePool.Path(p.Files()[0].Filename, p.Files()[0].Checksums)
+	packagePool := files.NewPackagePool(c.MkDir())
+	p := NewPackageFromControlFile(s.stanza)
+	p.Files()[0].Checksums.Size = 5
 
-		list, err := p.DownloadList(packagePool)
-		c.Check(err, IsNil)
-		c.Check(list, DeepEquals, []PackageDownloadTask{
-			{
-				RepoURI:         "pool/contrib/a/alien-arena/alien-arena-common_7.40-2_i386.deb",
-				DestinationPath: poolPath,
-				Checksums: utils.ChecksumInfo{Size: 5,
-					MD5:    "1e8cba92c41420aa7baa8a5718d67122",
-					SHA1:   "46955e48cad27410a83740a21d766ce362364024",
-					SHA256: "eb4afb9885cba6dc70cccd05b910b2dbccc02c5900578be5e99f0d3dbf9d76a5"}}})
+	list, err := p.DownloadList(packagePool)
+	c.Check(err, IsNil)
+	c.Check(list, DeepEquals, []PackageDownloadTask{
+		{
+			File: &p.Files()[0],
+		},
+	})
 
-		err = os.MkdirAll(filepath.Dir(poolPath), 0755)
-		c.Assert(err, IsNil)
+	tmpFilepath := filepath.Join(c.MkDir(), "file")
+	c.Assert(ioutil.WriteFile(tmpFilepath, []byte("abcde"), 0777), IsNil)
+	p.Files()[0].PoolPath, _ = packagePool.Import(tmpFilepath, p.Files()[0].Filename, &p.Files()[0].Checksums, false)
 
-		file, err := os.Create(poolPath)
-		c.Assert(err, IsNil)
-		file.WriteString("abcde")
-		file.Close()
-
-		list, err = p.DownloadList(packagePool)
-		c.Check(err, IsNil)
-		c.Check(list, DeepEquals, []PackageDownloadTask{})*/
+	list, err = p.DownloadList(packagePool)
+	c.Check(err, IsNil)
+	c.Check(list, DeepEquals, []PackageDownloadTask{})
 }
 
 func (s *PackageSuite) TestVerifyFiles(c *C) {