Fix alias handling of get_content_id

[sfan5]

...also some miscellaneous cleanups in itemdef.cpp
fixes #9632

To do

This PR is a Ready for Review.

How to test

Put the following code in the top level of a mod, not inside a callback or chatcommand.

Test that the following code succeeds:

minetest.register_alias("aliastest1", "air")
assert(minetest.get_content_id("aliastest1") == minetest.get_content_id("air"))

Test that the following code throws an error:

minetest.register_alias("aliastest2", "asdf:asdf")
minetest.get_content_id("aliastest2")

[SmallJoker]

Works as expected.

[Treer]

Thanks for fixing that!

[HeyITGuyFixIt]

Why would using minetest.get_content_id to get the id of a non existing node result in an error? Wouldn't it be better to simply return with nil, or is there a better way to check for a node's existence? The moresnow mod uses it assuming it will return a value and never an error.

[Treer]

@HeyITGuyFixIt The PR that introduced that change to get_content_id is #9458, and they make the argument for the behaviour change there.

My cloudlands mod was also using it to check for whether nodes exist, so I had to update the lines to this:
if minetest.registered_nodes[name] ~= nil then result = minetest.get_content_id(name) end

Obviously I had found the old behaviour useful, but I must admit that if you ignore the alias handling bug it uncovered (i.e. what this PR fixes) very few mods seem to have been [incorrectly] broken by the new get_content_id behaviour. Checking for the existence of nodes seems to be a surprisingly uncommon practice in Minetest mods - or was being done with the registered_nodes table.