Warn of dependence on plugin fileinstall

[wxtim]

Summary

Cylc cannot validate ahead of plugin (cylc rose) file installation if validation relies on files installed.

Example

# rose-suite.conf
[file:foo.rc]
source="/some/path/foo.rc"

# flow.cylc
%% include foo.rc

Built update at http://localserver..../cylc-doc/8.3.0.20240410A/html/7-to-8/cheat-sheet.html#admonition-0

Requirements check-list

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.

[wxtim]

Decided not to do this in offline convo