No way to override crates in [patch] section

[jethrogb]

#228 always adds a [patch] section to Xargo's Cargo.tomls, but there is no way to override the crate it points to. It always uses the crate it find in rust-src, even if std or core itself is built using a path or git source. This leads to two different, incompatible, core crates in the dependency tree.

See #231 for more issues with git sources.

I've come up with 3 different approaches to fix this, not sure which one is best:

1. If there is a std crate in the blueprint, compute ../tools/rustc-std-workspace-core from that path (or use the same git) and use that as the [patch].

2. If there is a std crate in the blueprint, find the spec of the core dependency (how would this work if std is a git dependency?), then generate a rustc-std-workspace-core crate in the tempdir with that core. Then, use that temp crate as the [patch].

3. Use the existing logic from Fix for latest nightly: need to add a patch section #228 to generate the [patch] by default. Let users specify their own [patch]es in Xargo.toml

[bjorn3]

+1 for letting users specify their own patches. Because I have to patch compiler_builtins I moved away from xargo in rust-lang/rustc_codegen_cranelift@1e5c6bc.

[jethrogb]

I added support for specifying [patch] in #235, but I still think it's weird/annoying you have to use that facility when specifying a git/path dependency for std and friends.