Providing CPLs via `-i` on the command line is duplicative.

[radford-for-smpte]

Th AssetMap must contain a URL that points to any and all CPLs contained in an IMP. Therefore providing a URL to the CPL is duplicative since a URL is already provided to the AssetMap.

Recommend the following command line syntax:

ffmpeg -i <assetmap-url> ...

ffempeg should:

1. Resolve & parse the AssetMap
2. search for all <Asset> elements that contains <PackingList>true</PackingList>
3. Resolve all URLs to the PackingLists.
4. Search each PackingList for <Asset> elements that contain a <Type> element with a value application/xml
5. For each, resolve the corresponding URL found in the AssetMap, determine if it’s a CPL, and process it.

When multiple CPLs are discovered, ffempg should treat each CPL exactly how it handles the case of multiple input streams provided with multiple -i options.