DevTools: Introduce module initializers

For all lazy-loaded modules and workers, _module.js files are introduced, which
importScript() all scripts for a particular module.

This allows to:
- get rid of importScript() calls intertwined with meaningful code,
- simplify module loading, since Runtime does not need to read the
  module descriptor to determine the resource names.

This approach is similar to how we implement
the CSS loading in inspector.css.

R=pfeldman, vsevik
BUG=391566

Review URL: https://codereview.chromium.org/451713002

git-svn-id: svn://svn.chromium.org/blink/trunk@179948 bbb929c8-8fbe-4397-9dbb-9b2b20218538