[Bug 79151] support global clues

[Henrik Nilsen Omma]

Public bug reported:

Some clues are very common across all packages. You may want to search
for bugs with core dumps, screenshots, filed by a certain person,
whatever.

This can be solved by inheritance into a bug clue file, but that
requires that file to exist and the include statement to be there. With
truly global clues I could type:

bughelper -c g_core_dump gok

to find all the bugs in which gok caused a core dump, without there even
being a clue file for gok. a global.info clue file would suffice.

** Affects: bughelper (upstream)
     Importance: Undecided
         Status: Unconfirmed

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support global clues
https://launchpad.net/bugs/79151