Allow pub_file (now pub_file_info) to be a dictionary of pub_vars

[oscarlevin]

This is an add on to the draft PR of @ascholerChemeketa (#2403) which would allow the CLI or other front ends to be in charge of maintaining the publisher variables dictionary.

[rbeezer]

#2403 is merged, so this could get sorted now. Thanks.

[oscarlevin]

Okay, this is all fixed up as a single commit. I think it is quite clever, but understand if you think it's too clever.

Summary of changes:

• The big idea: the pub_file variable that is in the signature of almost every function here is changing from a string (a path to the publication file) to either a string or dictionary.
  • If it is a string, then everything works as before.
  • If it is a dictionary, then one of the keys in the dictionary will be "publisher" with value the string-type path to the publication file.
• To remind us of this, variable name has changed from pub_file to pub_file_info.
• There are three ways we currently use the pub_file:
  1. Add it's path to stringparams. This is now done with the function add_pub_file_to_params() which checks if the type is a dictionary or string and does the right thing accordingly.
  2. Use the get_publisher_variable_report() to create a dictionary of pub_vars and then get_publisher_variable() to get the value of a particular variable. All instances of the former have been replaced with ensure_publisher_variables() which either does nothing (if already a dictionary) or calls get_publisher_variable_report().
  3. Use lxml to parse the publication file directly. This happens in two places: as part of the braille conversion and part of the get_managed_directories() function. I think in both cases these could be refactored to use the get_publisher_variable() function, but until then, we define pub_file to be the path appropriately.

Why all this? It is to address the Runestone use case of starting a worker to runs pretext, through the CLI's API. Previously this was a problem because as soon as the pub_vars dictionary was created, that would either persist between book builds, or (core) pretext would need to rebuild the dictionary for every step of building a book (generating each asset and building output). With this change, the CLI can create pub_file_info as a dictionary once for each book and use that in all the build/generate steps.