The directory in the tarball of vendored dependencies contains `name`,
which is by default set to `${pname}-${version}`. This adds an
additional attribute to permit setting the name to something of the
user's choosing.
Since `cargoSha256`/`cargoHash` depend on the name of the directory of
vendored dependencies, `cargoDepsName` can be used to e.g. make the
hash invariant to the package version by setting `cargoDepsName =
pname`.
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