Refactoring: use checksums instead of MD5 for pool/published

This is related to #506

As a first step, don't pass MD5 explicitly, pass checksum info object,
so that as a next step we can choose which hash to use.

There should be no functional changes so far.

Next step: stop returning explicit paths from public package pool.

deb/package_files_test.go

@@ -30,7 +30,7 @@ func (s *PackageFilesSuite) SetUpTest(c *C) {
 
 func (s *PackageFilesSuite) TestVerify(c *C) {
 	packagePool := files.NewPackagePool(c.MkDir())
-	poolPath, _ := packagePool.Path(s.files[0].Filename, s.files[0].Checksums.MD5)
+	poolPath, _ := packagePool.Path(s.files[0].Filename, s.files[0].Checksums)
 
 	result, err := s.files[0].Verify(packagePool)
 	c.Check(err, IsNil)