Optimize custom command full-path dependency lookup

In the common case of custom command dependencies specified via full
path optimize the implementation of GetSourceFileWithOutput using a
(hash) map.  This is significantly faster than the existing linear
search.  In the non-full-path case fall back to the existing linear
suffix search.
Merge topic 'optimize-custom-command-dependencies' into next

2268c41 Optimize custom command full-path dependency lookup