[Issue 18423] rdmd incorrectly assumes that the D compiler outputs dependency info to stdout

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18423

--- Comment #1 from Joseph Rushton Wakeling <joseph.wakeling at webdrake.net> ---
For context, this issue was observed while trying to update rdmd's integration
tests to use gdmd:
https://github.com/dlang/tools/pull/307#issuecomment-363564781

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