Out-of-tree testing is broken with the most recent update from
rust-lang/rust because it makes `compiler-builtins` depend on `core` by
path, which isn't usually available. In order to enable testing outside
of rust-lang/rust, add a new crate `builtins-shim` that uses the same
source as `compiler-builtins` but drops the `core` dependency. This has
replaced `compiler-builtins` as the workspace member and entrypoint for
tests.
`compiler-builtins` uses `public-test-deps`, `libm` uses
`unstable-public-internals`. Consolidate these under the `libm` name.
Once compiler-builtins is no longer published, this feature can probably
be dropped.
Also switch to `dep:` syntax for features that enable dependencies.
In order to disambiguate things now that libm is part of the
compiler-builtins repository, do the following:
* Mention libm in LICENSE.txt
* Clarify the default license for crates other than libm and
compiler-builtins
* Add an explicit license field to Cargo.toml for all other crates
This crate doesn't need to be a default member since it requires the
opposite settings from everything else. Exclude it from the workspace
and run it only when explicitly requested.
This also makes `cargo t --no-default-features` work without additional
qualifiers. `--no-default-features` still needs to be passed to ensure
`#![compiler_builtins]` does not get set.
compiler-builtins needs doctests disabled in order for everything to
work correctly, since this causes an error running rustdoc that is
unrelated to features (our `compiler_builtins` is getting into the crate
graph before that from the sysroot, but `#![compiler_builtins]` is not
set).
We can also remove `test = false` and `doctest = false` in
`builtins-test` since these no longer cause issues. This is unlikely to
be used but it is better to not quietly skip if anything ever gets added
by accident.
