d_library: path error when including source from dep libs.

Example:

```starlark
# d/mainlib/BUILD
d_library(name="mainlib",
  deps=["//3rdparty/a"])
```

```starlark
# 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.
