[gulp 4] allow forward references to tasks

Currently, forward references to tasks do not work in gulp 4 (and gulp 3, but I don’t care about that):

gulp.task('default', gulp.series('clean', 'build', 'watch', 'server'));

gulp.task('clean', function () {
    // do something
});
...

This results in AssertionError: Task never defined: clean.

From a stylistic standpoint, many people prefer to place the broadest tasks first and then define component tasks later, since this is how most people think and design code. Also the current ‘bottom up’, Forth-like restriction is no longer necessary considering how gulp 4 works.

From a design standpoint, since tasks are interchangeable with functions now, and since functions (like most stuff in Javascript) can be referenced from anywhere within the current scope regardless of where it was declared, to be consistent tasks should work the same way.

[mryellow]

This makes migration to v4 much more time-consuming than it needs to be.

what’s the point of a registry if everything needs to be defined in order?

I’m defining tasks here, with names as strings. I expect them to exist.

My fear with that approach is that users will see in G4 a product that is too different from the product they’re already used to and like,

Upon discovering this, I’m ready to stick with v3 for the next 6 decades.

[yocontra]

I kind of agree with @alvint here - what’s the point of a registry if everything needs to be defined in order? It seems super inflexible. If construction of the function is really a perf problem why can’t we use caching to only construct once on first run?

Possible use case for this is splitting tasks across multiple files, which with the current impl would all have to be loaded in the right order.