The previous code validated the manifests exist within the file system,
which only works when a chart is unpacked, meaning children charts
stored as tgz's in the parent chart's `charts/` directory don't exist as
individual files in the filesystem, causing validation to file.

Instead, we validate against the paths within the chart itself rather
than the filesystem, handling both chart.tgz's and charts that are
unpacked as directories in the chart/ directory.