support comma-separated paths in Go-Import-Path (#75)

Some packages can be imported from several import paths.
XS-Go-Import-Path can reflect this by using a comma-separated list
of known import paths for the package.

This patch updates make.go's existing package detection mechanism to
support this.

The pkgs struct field ImportPath is renamed to XSGoImportPath to avoid
confusion because it could be that XSGoImportPath is not a valid import
path if it contains commas.

For example, the following format would now be valid:
```
XS-Go-Import-Path: github.com/go-mgo/mgo,
                   gopkg.in/mgo.v2,
                   labix.org/v2/mgo,
                   launchpad.net/mgo
```

Author: aviau

search.go

@@ -26,9 +26,9 @@ func getGolangBinaries() (map[string]string, error) {
 		return nil, fmt.Errorf("unexpected HTTP status code: got %d, want %d", got, want)
 	}
 	var pkgs []struct {
-		Binary     string `json:"binary"`
-		ImportPath string `json:"metadata_value"`
-		Source     string `json:"source"`
+		Binary         string `json:"binary"`
+		XSGoImportPath string `json:"metadata_value"`
+		Source         string `json:"source"`
 	}
 	if err := json.NewDecoder(resp.Body).Decode(&pkgs); err != nil {
 		return nil, err
@@ -37,7 +37,10 @@ func getGolangBinaries() (map[string]string, error) {
 		if !strings.HasSuffix(pkg.Binary, "-dev") {
 			continue // skip -dbgsym packages etc.
 		}
-		golangBinaries[pkg.ImportPath] = pkg.Binary
+		for _, importPath := range strings.Split(pkg.XSGoImportPath, ",") {
+			// XS-Go-Import-Path can be comma-separated and contain spaces.
+			golangBinaries[strings.TrimSpace(importPath)] = pkg.Binary
+		}
 	}
 	return golangBinaries, nil
 }