Rename the `public-test-deps` feature to `unstable-public-internals`

`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.
8 files changed
tree: e87dbc39cfb97faff0bcccb6a38bd9ff07cf5c81
  1. library/