Findlib library `requires` for non-empty predicates

[sim642]

Currently findlib library dependencies are looked up with the empty predicates list, but other possibilities may exist:

• https://sourcegraph.com/search?q=context%3Aglobal+file%3A%5EMETA%24+requires%28&patternType=standard&case=yes&groupBy=repo
• https://cs.github.com/?scopeName=All+repos&scope=&q=path%3A%2F%5EMETA%24%2F+%22requires%28%22

To get a complete listing for all predicates, the META files need to be manually traversed: http://projects.camlcity.org/projects/dl/findlib-1.9.6/doc/ref-html/lib/Fl_metascanner.html#TYPEpkg_definition.