New xgettext method to i18n module

[bruchar1]

This method call xgettext to extract translatable
string from source files into a .pot translation template.

It differs from a plain CustomTarget in three ways:

• It accepts build targets as sources, and automatically resolves source
  files from those build targets;
• It detects command lines that are too long, and writes, at config
  time, the list of source files into a text file to be consumed by the
  xgettext command;
• It detects dependencies between pot extraction targets, based on the
  dependencies between source targets.

This does the specific thing I need for my project, that I was trying to implement in a more generic way in #12272 and #11822.

[eli-schwartz]

Since the underlying goal as I understand it is essentially "i18n.gettext() is not suitable for my use case because it depends on POTFILES but I would like to calculate the list from meson.build", it feels a bit like a more appropriate solution here could be to teach i18n.gettext() to take an optional override of files to extract from (and e.g. resolve build targets by recursing into their source filelist, if desirable) -- and then generate a POTFILES for you, basically.

[bonzini]

and then generate a POTFILES for you, basically.

I understood that he also wants to generate multiple .pot files, so that would not be a single POTFILES.

[jpakkane]

API-wise LGTM. Are there still open implementation questions?

[jpakkane]

The "fixes for mypy" commit should be folded into the other commits.

[eli-schwartz]

It looks like the mypy fixes aren't actually related, just underlying issues that the proposed enhancement coincidentally hits. It's fine as a separate commit. The second commit I'd probably mention why it's being split out -- just say "we want to call it from a module in a followup commit".

[eli-schwartz]

The API seems fine to me, I guess. I haven't really taken a very close look at the implementation and how it interacts with targets, so my review doesn't cover that. There are some minor details I did notice though.