Track `panic` in Unit early.

This is an alternate solution for #5445. It ensures that `panic` is cleared in the Profile for "for_host" targets (proc-macro/plugin/build-scripts) and dependencies. This helps avoid unnecessary recompiles in some situations (though does add extra units in some situations, see below).

Some examples where extra rebuilds are now avoided:
* A workspace with a dependency shared with normal and build-deps.  `build --all` should build everything, and then `build -p foo` was causing a recompile because the shared dep was no longer in the `used_in_plugin` set. Now it should not recompile.
* `panic=abort`, with a shared dependency in build and dev, `build` would cause that shared dependency to be built twice (exactly the same without panic set). Now it should only build once.

Some examples where `panic` is now set correctly:
* `panic=abort`, with a binary with a shared dependency in build and normal, `test` would cause that shared dependency to be built twice (exactly the same without panic set).  Now it is still built twice, but the one for the normal (bin) dependency will correctly have `panic` set.

Some examples where new units are now generated:
* `panic=abort`, with shared dependency between normal and proc-macro or build. Previously the shared dependency was built once with `panic=unwind`. Now it is built separately (once with `panic`, once without). I feel like that this is more correct behavior — that now the normal dependency avoids adding landing pads.

   For `panic=abort` cross-compiling, this makes no difference in compile time since it was already built twice.

Additional notes:
- I left build-scripts with the old behavior where `panic` is cleared for it and all its dependencies. There could be arguments made to change that (#5436), but it doesn't seem important to me.
- Renamed and refactored `ProfileFor` to `UnitFor`. I expect this API to continue to evolve in the future.

Closes #6143, closes #6154.