Make package.name optional

[epage]

Problem

The primary motivation for this is to close the gap between Cargo.toml and the cargo script experiment (#12207).

For Cargo.toml, package.name is required

For "cargo script", package.name is defaulted to a slug of the file stem.

Proposed Solution

Like with #9829, making this field optional would reduce the specialized documentation around "cargo script". The default for Cargo.toml would be a slug of the parent directory.

Notes

I think there are some other concrete cases that could potentially benefit from this, like rust-lang/rfcs#3452 though I overall see this rarely being relied upon.

[est31]

I'm not sure we'd be able to reduce documentation much here, as the logic to take the filename would not be suiting for Cargo.toml, so we'd have to document that differing logic. We would be able to reduce the overhead of writing existing Cargo.toml files though, which would be a benefit. I'd propose taking the name of the directory the Cargo.toml file is inside.

[epage]

Oops, forgot to specify the parent directory.

While that technically is still a difference, the general concept is the same which makes it easier to teach.