parser: Improve performance of expanding strings

parser: Improve performance of expanding strings

By using pakfire_string_replace, all occurrences of a variable will be
replaced at once until no variables are being left.

It can still happen, that the same variable is being matched later again
(multiple times even), but it looks to me like adding any caching
mechanism adds a lot more overhead.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>