bp2build: add support for planting unresolved symlinks in the symlink forest.

In the isDir check, there's a possibility that the Stat check fails
because the path is an unresolved symlink. Verify it with Lstat (which
would succeed, since it doesn't follow links), and treat it like a file
if Lstat succeeds.

Test: new integration test
Fixes: 232370097
Change-Id: I9807ca363a5dbdc20639b489b54627bd2cc1ca60