Remove one more unneeded hashtable

[Marco Diego Aurélio Mesquita]

Hi Devs!

Please consider the attached patch.

The old algorithm for dependency checking in pkg-config used a boolean
member on the Packages structure (in_requires_chain) to check if a
package was already on the "requires chain". The problem with the old
algorithm was that such member was reset at the end of
recursive_fill_list. Matthew Hanna fixed such problem by using a
hashtable (in 908cdd), but this can actually be done the old way
(without any hashtable) if the boolean member is reset at the right
places.

The attached patch brings such a member back, resets it on the right
places and removes the need for another hashtable.

Please review it.
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