Auto merge of #64823 - cuviper:min-std, r=Mark-Simulacrum

Auto merge of #64823 - cuviper:min-std, r=Mark-Simulacrum

minimize the rust-std component

This changes the `rust-std` dist component to only include the artifacts of compiling the `libstd` step, as listed in `.libstd.stamp`. This does include `test` and `proc-macro` as well. The remaining _unstable_ libraries that are built as part of `rustc` are packaged into a new `rustc-dev` component, intended for use in the development of closely related tools (clippy, miri, rls).

Here are the component sizes from the [try build](https://dev-static.rust-lang.org/dist/2019-10-07/index.html):

| Name | Size
| --- | ---
| rust-std-nightly-x86_64-unknown-linux-gnu.tar.gz | 23.94 MiB
| rust-std-nightly-x86_64-unknown-linux-gnu.tar.xz | 17.4 MiB
| rustc-dev-nightly-x86_64-unknown-linux-gnu.tar.gz | 182.03 MiB
| rustc-dev-nightly-x86_64-unknown-linux-gnu.tar.xz | 157.91 MiB

Fixes #61978
Fixes #62486