[no squash] Get rid of the last remaining sync. HTTP requests on the main thread

[grorp]

Building upon the work in #13551 and #14412, this PR aims to make sure that there will never be ANRs related to HTTP requests again.

• Commit 1 moves the last remaining HTTP request from the main thread to core.handle_async. To reduce the amount of changes to existing code and to avoid callback hell, it uses coroutines.

  The HTTP request in question is the one that fetches a dependency list from CDB before attempting to install a package. While it's usually very fast, it can take long if you have a bad internet connection or if it times out.

• Commit 2 removes the ability to execute synchronous HTTP requests on the main thread from httpfetch.cpp. This doesn't break backwards compatibility because fetch_sync is only available in the mainmenu. Since any synchronous HTTP request on the main thread is a bug, we should avoid introducing one again in the future.

To do

This PR is a Ready for Review.

• Investigate whether more code can be removed from httpfetch.cpp.
  There is probably more to remove, but I fear that I don't know the code well enough to avoid accidentally breaking things.

How to test

Verify that Minetest doesn't freeze for a short while anymore after you press the install button on a package.

Try to install lots of different packages from CDB. Verify that dependencies are resolved correctly and that packages are installed correctly.

[rubenwardy]

Might be interesting to add a coroutine http api

[grorp]

Implemented in 066e07c to give the idea a try.

Differences:

• core.parse_json runs on the main thread again. Previously, this PR moved it into core.handle_async.
• The difference between the master branch and this PR is smaller than before.
• The overall amount of code is still exactly the same.
• Is this approach more ergonomic? I don't know.

What do you think?

EDIT: I've come to the conclusion that I don't like it, also because I noticed that the old version can be made even shorter.

[sfan5]

Maybe I just don't understand this resumer-pattern, but would await (like here) be a suitable replacement?
Just a suggestion ofc.

[grorp]

When using your await function, the problem I have is that the final callback (the one passed into contentdb.has_hard_deps / contentdb.resolve_dependencies by the API user) is not called. To make this work, the value returned by coroutine.resume here

minetest/games/devtest/mods/unittests/init.lua

Line 38 in 6303334

coroutine.resume(co, ...)

would somehow have to get back to the final callback, which brings me back to my "resumer" pattern.

This is my first time working with coroutines. There might of course be a better approach that I haven't seen.