Rename $layout to __layout, etc

[Rich-Harris]

Closes #1149.

• disallow files/directories with leading __, so that we can add more special files in future without breakage
• error on $layout etc
• create a utility for renaming files? npx @sveltejs/kit-rename-files or something?

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[Conduitry]

Amusingly, I think the __tests__.js files are now tripping the rule about not being able to start a file with two underscores unless it's one of the known ones.

[Rich-Harris]

whoops

AFK for a bit, will fix later if no-one beats me to it

[frederikhors]

This change makes working with Go a nightmare because of this: golang/go#43854.

Can we add a config option for this, @Rich-Harris?

[feedmeapples]

@Rich-Harris any updates on the config option?

[rohfle]

As a workaround, I added multiple paths and it worked fine with the sveltekit demo. Note the * to include paths beginning with . and _. The multiple paths are necessary because * includes these files in the immediate directory, but not in subdirectories.

//go:embed frontend/build/* frontend/build/_app/pages/* frontend/build/_app/assets/pages/*

Also note that files cannot be embedded from the parent directories of the go file where they are embedded. (no ../.. in the path)

[rohfle]

The all: path prefix for go:embed has also been recently merged in Go (https://go-review.googlesource.com/c/go/+/359413/)

cmd/go: add //go:embed all:pattern

[...]

After long discussions on #42328 and #43854, we decided to keep
the behavior of excluding . and _ files by default, but to allow the pattern
prefix 'all:' to override this default. This CL implements that change.

Note that paths like .git and com1 are still excluded, as they must be,
since they will never be packed into a module.

[frederikhors]

@rohfle I think it will be available from 1.18, right?

[rohfle]

@frederikhors the all: path prefix is merged in 1.18beta1