Make sure we do compute the best fit

While install calls upgrade which in turns calls a relaxed install on
its results, this doesn't make sure that the new results are at the best
available version. We do iterate here over the results to compute the
best set.

It also expands computeUpgrade with the possibility to selectively
choose which packages to upgrade and which not.

pkg/solver/decoder.go

@@ -146,6 +146,17 @@ func (assertions PackagesAssertions) Search(f string) *PackageAssert {
 	return nil
 }
 
+func (assertions PackagesAssertions) ToDB() pkg.PackageDatabase {
+	db := pkg.NewInMemoryDatabase(false)
+	for _, a := range assertions {
+		if a.Value {
+			db.CreatePackage(a.Package)
+		}
+	}
+
+	return db
+}
+
 func (assertions PackagesAssertions) Order(definitiondb pkg.PackageDatabase, fingerprint string) (PackagesAssertions, error) {
 
 	orderedAssertions := PackagesAssertions{}