Collection: automatic compile linked dependencies (recursive document hierarchy)

[ronaldtse]

We want to implement the changes to the inclusion attributes too:
https://github.com/metanorma/bipm-si-brochure/blob/5b6d6578a88022010769721e9178167054e7ce61/sources/si-brochure-bilingual.yml#L43-L67

When specifying a collection, it is cumbersome to specify the "path of the rendered output file". Most likely, the user has not compiled the dependent files used by the collection.

While it is possible to specify in a site generate config file, the order of the dependencies (i.e. list out dependent files first, then the collection), as done in metanorma/bipm-si-brochure#186, it is not ideal.

Instead, we want to automatically resolve all dependencies IN ADVANCE, and then compile the documents in the correct order.

e.g.

manifest:
  documents:
    - source: si-brochure-fr.adoc
    - source: si-brochure-en.adoc 

i.e.

brochure.xml
  |
  | (depends on)
  |
  +-- si-brochure-fr.adoc
  +-- si-brochure-en.adoc

Notice that the (intermediate) output location of the si-brochure-fr.adoc file (e.g. somepath/si-brochure-fr.xml) does not matter as long as the results will be provided at the output location.

Right now we are only allowing a one-level dependency but clearly this can go deeper into recursive levels.

The approach to do this is to have ProcessingTree, ProcessingNode classes to model this and then link the outputs back to the parent.

@abunashir can you please help implement this? Thanks.

Originally posted by @ronaldtse in #205 (comment)