Allow mirrors_predefined and mirrors_blacklist to reference files

[ajdunevent]

An enhancement idea would be to allow the variables that accept an array of repos to also accept an input file whose content is a correctly formatted mirrorlist.

This would be particularly beneficial for mirrors_predefined because it would allow the input to be a predefined/fallback list such as a curated mirrorlist from pacman's default or a sorted mirrorlist like reflector's output.

[nroi]

Interesting idea, reflector is a tool that people already use and trust, so it's totally reasonable if people want to re-use that functionality in flexo.

I currently don't have much time to implement it myself, but if someone else wants to do it, PRs welcome.

I haven't thought it out in detail yet, but for the config file, I'm guessing it could look something like this:

the mirror_selection_method has a new entry, this could be e.g. from_file:

# The selection method to choose a mirror. Valid values are:
#   "auto": Flexo will attempt to find suitable mirrors automatically.
#           With this method, performance tests are run on the official mirrors
#           to select only sufficiently fast mirrors.
#   "predefined": To only choose the mirrors defined for the variable
#                 mirrors_predefined (see below).
#  "from_file": bla bla read @ajdunevent  idea above 
mirror_selection_method = "from_file"

and then introduce a new variable:

mirrors_file = "/path/to/mirrorlist