Prevent addition of duplicate path prefixes.

I think I understand this now; the failure comes from here:

https://golang.org/src/cmd/go/vcs.go#L818

and makes it so one can't both host:
- github.com/foo
- github.com/foo/bar

which would have to clone the latter inside of the former.

Fixes #1.

storage.go

@@ -2,9 +2,19 @@ package vain
 
 import (
 	"errors"
+	"strings"
 	"sync"
 )
 
+func Valid(p string, packages []Package) bool {
+	for _, pkg := range packages {
+		if strings.HasPrefix(pkg.Path, p) {
+			return false
+		}
+	}
+	return true
+}
+
 type MemStore struct {
 	l sync.RWMutex
 	p map[string]Package