Use splitter instead of split in getopt

[dkorpel]

Split off from #8113
split eagerly returns a GC-allocated array of slices. Since scope only applies to the top level, this returned array escapes the input string as far as dip1000 is concerned.

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[thewilsonator]

grep -nr 'import std.algorithm : ' $(find etc std -name '*.d') ; test $? -eq 1
--
  | std/getopt.d:1109:    import std.algorithm : splitter;

I think you need to import std.algorithm.mutation : splitter; or whatever the submodule is.