dev-dependencies on workspace member cause cyclic dependency issues

Note: We (rust-analyzer) consider this issue resolved in terms of what we can do here.

This issue is spread across several others, so let's make one big one out of it to better track it.

https://github.com/rust-lang/rust-analyzer/issues/2414#issuecomment-561213070

Yeah, the problem is that a crate can have a dev-dep onto itself:

[package]
name = "foo"
version = "0.1.0"
edition = "2018"

[dev-dependencies]
foo = { path = "." }

The solution here is to start "duplicating" crates when lowering cargo-metada output to CrateGraph. In the above example, we should construct two foo crates, one with cfg(test) and one without, and make the required depednency edge.

Issues in question:
https://github.com/rust-lang/rust-analyzer/issues/8330
https://github.com/rust-lang/rust-analyzer/issues/2414
https://github.com/rust-lang/rust-analyzer/issues/12407
https://github.com/rust-lang/rust-analyzer/issues/9574
https://github.com/rust-lang/rust-analyzer/issues/11410

Start by reading the linked issues 8330, 2414, 12407, 9574, and 11410, along with the referenced discussion about lowering cargo metadata to CrateGraph. The issue names no files or tests, and rust-analyzer considers its part resolved, so done cannot be defined without identifying a remaining concrete case.