Supporting more build environments

[ef4]

• adds an alternative eager implementation for importSync that works in all ES module environments (at the cost of making some synchronous-but-lazy things become eager)
• refactored core so that parts of it are easier to consume from browser code without pulling in node-specific features
• updated app-js and fastboot-js metadata to a stricter form that requires all files by listed explicitly, not just the directories.
• made it easier for compat adapters to edit the output package.json, beyond just the ember-addon section.
• updated several places to keep explicit file extensions within the build, where in the past we were unnecessarily stripping them off