runtime: Change external plugin directory to "external-plugin"
runtime: Prepend the vimrc filename to the external plugin manifest

This is required to support multiple vimrcs with each having it's own set of
installed external plugins.
doc: Update external plugin documentation
doc: Add nvim_provider/external_plugin to doc Makefile
Merge PR #1504 'Change external plugin dir, manifest and improve docs'
Allow most tests to pass with nocp
Allow the rest of the tests to pass with nocp

Includes the change of one *.ok file that would have otherwise required
cpoptions+=n