Fix/remove some bad std::moves
(cherry picked from commit 8c049a9f044569ebda70231709f6f15d3073894a)
Change-Id: I720273378d2506a13883acee28abd096d099b0d4
This introduces the SourcePath type from lazy-trees as an abstraction
for accessing files from inputs that may not be materialized in the
real filesystem (e.g. Git repositories). Currently, however, it's just
a wrapper around CanonPath, so it shouldn't change any behaviour. (On
lazy-trees, SourcePath is a <InputAccessor, CanonPath> tuple.)
eldritch horrors