Improve error messages involving `derive` and `packed`.

There are two errors involving `derive` and `packed`.

```
`#[derive]` can't be derived on a `#[repr(packed)]` struct with type or const parameters
`#[derive]` can't be derived on a `#[repr(packed)]` struct that does not derive Copy
```
The second one overstates things. It is possible to use derive on a
repr(packed) struct that doesn't derive Copy in two cases.
- If all the fields within the struct meet the required alignment: 1 for
  `repr(packed)`, or `N` for `repr(packed(N))`.
- If `Default` is the only trait derived.

This commit improves things in a few ways.
- Changes the errors to say `$TRAIT can't be derived on this ...`.
  This is more accurate, because it's just $TRAIT and *this* packed
  struct that are a problem, not *all* derived traits on *all* packed
  structs.
- Adds more details to the "ERROR" lines in the test case, enough to
  distinguish between the two error messages.
- Adds more cases to the test case that don't cause errors, e.g. `Default`
  derives.
- Uses a wider variety of builtin traits in the test case, for better coverage.

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