Handle symlinks in isPackageBoundary

isPackageBoundary looks at ShouldKeepExistingFile before checking if
that directory contains a BUILD file or not. ShouldKeepExistingFile
should be complemented with a isSymlink check. This is necessary because
we copy all symlinks to the synthetic workspace, and the resolved
symlink might point to a directory containing a BUILD file.

This additional clause is redundant if the directory has been
allowlisted for keepExistingBuildFile (e.g. build/bazel, recursive)

Test: b build //bionic/libc:versioner-dependencies (top of stack)

Change-Id: I5b23262f89ea34a78de4ccade6d27e4c5dd95c2e