Add support for Package provides

Add "provides" field in packages (which affect both runtime and buildtime deps).
It replaces all the occurences in the deptree before solving, actually
allowing to swap packages and provide virtuals. Along with a mechanism
for package rename #25.

pkg/solver/decoder.go

@@ -168,12 +168,16 @@ func (assertions PackagesAssertions) Order(definitiondb pkg.PackageDatabase, fin
 	}
 
 	sort.Sort(unorderedAssertions)
-
 	// Build a topological graph
 	//graph := toposort.NewGraph(len(unorderedAssertions))
 	//	graph.AddNodes(fingerprints...)
 	for _, a := range unorderedAssertions {
-		for _, requiredDef := range a.Package.GetRequires() {
+		currentPkg := a.Package
+		for _, requiredDef := range currentPkg.GetRequires() {
+			if def, err := definitiondb.FindPackage(requiredDef); err == nil { // Provides: Get a chance of being override here
+				requiredDef = def.(*pkg.DefaultPackage)
+			}
+
 			// We cannot search for fingerprint, as we could have selector in versions.
 			// We know that the assertions are unique for packages, so look for a package with such name in the assertions
 			req := assertions.SearchByName(requiredDef.GetPackageName())
@@ -182,7 +186,7 @@ func (assertions PackagesAssertions) Order(definitiondb pkg.PackageDatabase, fin
 			}
 
 			// Expand also here, as we need to order them (or instead the solver should give back the dep correctly?)
-			graph.AddEdge(a.Package.GetFingerPrint(), requiredDef.GetFingerPrint())
+			graph.AddEdge(currentPkg.GetFingerPrint(), requiredDef.GetFingerPrint())
 		}
 	}
 	result, err := graph.TopSort(fingerprint)