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'
legacy tests: migrate test5
legacy tests: remove test21 files
legacy tests: implement :source helper method.
legacy tests: migrate test104
legacy tests: migrate test25
legacy tests: migrate test66
legacy tests: migrate test67
legacy tests: migrate test33
legacy tests: migrate test43
legacy tests: migrate test51
legacy tests: migrate test75
legacy tests: migrate test101
legacy tests: migrate test26
legacy tests: migrate test105
Merge PR #1420 'Migrate legacy tests (2nd batch)'