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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