Improve /api/v1/repos/issues/search by just getting repo ids (#15179)

Improve /api/v1/repos/issues/search by just getting repo ids (#15179)

/api/v1/repos/issues/search is a highly inefficient search which is unfortunately
the basis for our dependency searching algorithm. In particular it currently loads
all of the repositories and their owners and their primary coding language all of
which is immediately thrown away.

This PR makes one simple change - just get the IDs.

Related #14560
Related #12827

Signed-off-by: Andrew Thornton <art27@cantab.net>