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. Refs. !463 (closed) (comment 136995) (discussion)

Refs. swh-lister!466 (merged) (lister implementation change)

Refs. swh/meta#4979 (closed)