add content requested and refactor python cataloger to use it

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

syft/source/content_requester.go

@@ -0,0 +1,48 @@
+package source
+
+import "sync"
+
+type ContentRequester struct {
+	request map[Location][]*FileData
+	lock    sync.Mutex
+}
+
+func NewContentRequester(data ...*FileData) *ContentRequester {
+	requester := &ContentRequester{
+		request: make(map[Location][]*FileData),
+	}
+	for _, d := range data {
+		requester.Add(d)
+	}
+	return requester
+}
+
+func (b *ContentRequester) Add(data *FileData) {
+	b.lock.Lock()
+	defer b.lock.Unlock()
+	b.request[data.Location] = append(b.request[data.Location], data)
+}
+
+func (b *ContentRequester) Execute(resolver ContentResolver) error {
+	b.lock.Lock()
+	defer b.lock.Unlock()
+
+	var locations = make([]Location, len(b.request))
+	idx := 0
+	for l := range b.request {
+		locations[idx] = l
+		idx++
+	}
+
+	response, err := resolver.MultipleFileContentsByLocation(locations)
+	if err != nil {
+		return err
+	}
+
+	for l, contents := range response {
+		for i := range b.request[l] {
+			b.request[l][i].Contents = contents
+		}
+	}
+	return nil
+}

syft/source/file_data.go

@@ -0,0 +1,6 @@
+package source
+
+type FileData struct {
+	Location Location
+	Contents string
+}