NodeLoader: Rename checksums_computation to checksum_layout

NodeLoader as in {Content|Directory}Loader. Those can directly ingest respectively
a (remote) file (as Content) or a (remote) tarball artifact (as Directory). They are
currently listed by the nixguix lister. Depending on the checksum_layout (standard,
nar), they will compute checksums differently. With "standard" checksum layout, they are
computing usual "swh" checksums (sha1, sha1_git, ...) and validate them. With "nar"
checksum layout, they will compute the "nar" checksums and store those (once validated)
as ExtID.

This keeps the node loader construtor retro-compatible with the previous version. It
still deals with `checksums_computation` as `checksum_layout` and falls back to
"standard" layout when nothing is provided.

Refs. swh/meta#4979