Export templates distributed as modular package

[CatMeowByte]

One gigabyte export templates per version, including patch version.
Means updating godot also have to redownload the template, even when half of it is almost never used.
The dependency that godot need is bigger than the godot itself.

In my personal case i mostly just need the Web export. i dont need the whole gigabyte of export template.

I imagine making it modular or split so when i export the web it will show

Web export template not installed, Install for Web

And so on for when i add the export options for other platform

Optionally to have

Install for All Platform

Which have the same behavior as the current implementation

Then it will only get the required export template for the appropriate version, its fast, its small, and only do whats necessary.

I assume it wouldnt be much of a difference other than making separate tpz file and upload it in the archive. besides, it cross platform anyway, godot maintainer only had to build it once to generate those module file like how it always been, just packaged differently.

[Calinou]

Thanks for the proposal! Consolidating in #647.