Dpkg::BuildFlags: Fix strip method to always work with duplicates

The regex was not correctly matching at the beginning of the line with
leading spaces, which could be left by a previous replacement from
the same s/// operator. Instead switch to split the flag value and
filter based on a hash, which means we do not need to care about such
space issues anymore.

This change should not change the semantics for space separated options
in the flags, as the code was already splitting the passed values on
spaces, and then remapping on the entire flag value.

Improve strip unit tests to cover all these cases.

Closes: #1028044