Fix: (org-ql-completing-read) :query-prefix prevented selection

COMPLETING-READ, when used with its REQUIRE-MATCH argument, would not
allow a result to be selected when ORG-QL-FIND was called with its
QUERY-PREFIX argument.

This change moves where the query is modified to include the prefix,
putting it into the RUN-QUERY function instead of the COLLECTION
function.  It seems to solve the problem, and the function seems to
still work when used without QUERY-PREFIX.  However, due to the
complexity of all this (we really need a SELECTING-READ API), it
wouldn't surprise me if something is later found to have been broken
by it...

Fixes #351.

Reported-by: Daniel Fleischer <https://github.com/danielfleischer>

org-ql.info

@@ -1033,6 +1033,10 @@ File: README.info,  Node: 072-pre,  Next: 071,  Up: Changelog
      ‘org-ql-find’).
    • Query parser ignores leading whitespace (e.g.  preventing errors
      while the user is typing a query into ‘org-ql-find’).
+   • Use of ‘org-ql-find’ with ‘:query-prefix’ argument prevented
+     selection of results.  (#351
+     (https://github.com/alphapapa/org-ql/issues/351).  Thanks to Daniel
+     Fleischer (https://github.com/danielfleischer) for reporting.)
 
 
 File: README.info,  Node: 071,  Next: 07,  Prev: 072-pre,  Up: Changelog
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