Flatten foreach calls that produce 2d lists

In make, the result of $(foreach $(x),$(y),$(foreach $(z),(w),a))
is a regular list, but in Starlark it's a list of lists. Flatten
the results of foreach expressions where each element is a list
so that they're regular lists of strings again.

Bug: 226974242
Test: go test
Change-Id: I3210d409aba0d807a5890e341ab1e0c0478f5930