Make mill-build folder opt in via import $meta._

[lihaoyi]

Fixes #2461

For now it remains hard-coded as mill-build/, for simplicity. Given the mill- prefix, it's unlikely to collide with user-defined folders. Making it configurable would open another can of worms where the import $meta.foo can now collide with an object foo, which is very unintuitive.

For now, we just parse the script files one additional time in MillBuildBootstrap.scala. This is a bit wasteful, but is probably fast enough for now, and we're already pretty sloppy parsing everything twice in MillBuildRootModule#scriptSources and MillBuildRootModule#parseBuildFiles, so parsing things three times isn't the end of the world. We can look into optimizing it in future if necessary

[lihaoyi]

Seems the integration tests need to be tweaked abit since we're no longer unconditionally going through all levels of meta-builds.

[lefou]

I think the $mill.build is a bit unfortunate. Although the $mill.build somehow corresponds to mill-build, once we want to support a custom name, it will no longer match as nice. Just because it already contains two segments.

I like the originally proposed $meta better, as it can support a custom name when given, e.g.

import $meta.`my-mill-build`

but also use the default name when used without any extra segment like

import $meta // short form of import $meta.`mill-build`.

[lihaoyi]

import $meta isnt valid syntax, but `import $meta._' works.

[lefou]

Looks good to me!