Only return the meta for requested package

the go tool currenty has a pathology where it mistakingly claims it
can't clone a repo because it checks prefix by bytes, not by splitting
the path on slash.

Once https://github.com/golang/go/issues/15947 comes out (go1.8) then go
tools will be able to handle being provided with valid but overlapping
packages.

For now though we'll just return meta for the requested package.

Change-Id: Ie5026e7d5c1377ff7d2c2140b21f9b745af69764

db.go

@@ -1,6 +1,7 @@
 package vain
 
 import (
+	"database/sql"
 	"fmt"
 	"log"
 	"net/http"
@@ -100,6 +101,12 @@ func (db *DB) PackageExists(path string) bool {
 func (db *DB) Package(path string) (Package, error) {
 	r := Package{}
 	err := db.conn.Get(&r, "SELECT * FROM packages WHERE path = ?", path)
+	if err == sql.ErrNoRows {
+		return r, verrors.HTTP{
+			Message: fmt.Sprintf("couldn't find package %q", path),
+			Code:    http.StatusNotFound,
+		}
+	}
 	return r, err
 }