RFR: Switch to luvit-loader

[creationix]

In order to make the luvit ecosystem more friendly with the existing lua ecosystem, luvit's custom require is being replaced with a loader plugin for the native require. This PR ports lit to this new system.

• Added luvit-loader.lua to the root. This adds a new loader that implements the old luvit require search paths as a loader function. The main difference is files are loaded using plain loadfile or loadstring without setting a custom environment and injecting fake globals.
• Lit metadata for single-file modules is now in a special comment instead of setting properties on an assumed global exports table.
• Exports value is always returned since there is no longer a global exports table.
• Lit's auto-config, auto-core, and command systems were changed slightly to never yield during a require call since we're using real require now.

This depends on changes to luvi (removing the luvit/require hook) and changes to the git submodule. Look for the luvly branches of both those projects.

To test this, make sure to use a custom luvi with the proposed changes from it's luvly branch.

[creationix]

Still left to do.

• Add support in lit to read the new metadata format.
• Add fallback support in loader to look in bundle if there is a bundle and file wasn't found on disk.
• Add support in loader to load binary modules.
• Add module caching in loader. I don't think lua does this for us.
• Use realpath in libuv to resolve paths for better caching.

[creationix]

Updated luvi is being published. Most the TODO items regarding luvit-loader functionality can't be tested in lit since it doesn't use those features. I'll punt them for the next step integrating with luvit.

[rphillips]

+1