d_library: path error when including source from dep libs.

[DavidYKay]

Example:

# d/mainlib/BUILD
d_library(name="mainlib",
  deps=["//3rdparty/a"])

# 3rdparty/a/BUILD
d_library(name="a",
  srcs=["src/a.d"])

In this case, DMD will fail, expecting to find 3rdparty/a/src/a.d on the local path.