Add new `redistribute()` directive (#20185)

Some packages can't be redistributed in source or binary form. We need an explicit way to say that in a package.

This adds a `redistribute()` directive so that package authors can write, e.g.:

```python
    redistribute(source=False, binary=False)
```

You can also do this conditionally with `when=`, as with other directives, e.g.:

```python
    # 12.0 and higher are proprietary
    redistribute(source=False, binary=False, when="@12.0:")

    # can't redistribute when we depend on some proprietary dependency
    redistribute(source=False, binary=False, when="^proprietary-dependency")
```


To prevent Spack from adding either their sources or binaries to public mirrors and build caches. You can still unconditionally add things *if* you run either:
* `spack mirror create --private`
* `spack buildcache push --private`

But the default behavior for build caches is not to include non-redistributable packages in either mirrors or build caches.  We have previously done this manually for our public buildcache, but with this we can start maintaining redistributability directly in packages.

Caveats: currently the default for `redistribute()` is `True` for both `source` and `binary`, and you can only set either of them to `False` via this directive.

- [x] add `redistribute()` directive
- [x] add `redistribute_source` and `redistribute_binary` class methods to `PackageBase`
- [x] add `--private` option to `spack mirror`
- [x] add `--private` option to `spack buildcache push`
- [x] test exclusion of packages from source mirror (both as a root and as a dependency)
- [x] test exclusion of packages from binary mirror (both as a root and as a dependency)

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