Fall back on require.resolve to find plugins.

When loading plugins, this means the local and global node_modules directories
will be searched after the project and plugin/ directories.

This change facilitates the independent distribution of plugins using npm.
Update manual to discuss third-party plugins.

Move the discussion of individual plugins into the 'Server plugins'
section (to match the table of contents), and insert a subsection about
distributing third-party plugins at the end of the section.