[feature] Expose `self.dependencies` in all methods of conanfile called after `validate()`

- [x] I've read the [CONTRIBUTING guide](https://github.com/conan-io/conan/blob/develop/.github/CONTRIBUTING.md).

In https://docs.conan.io/en/1.51/reference/conanfile/dependencies.html, there is:

> At the moment, this information should only be used in `generate()` and `validate()` methods. Any other use, please submit a Github issue.

So I'm submitting an issue to extend self.dependencies to other methods :)

Actually in this PR of `tinyspline` recipe https://github.com/conan-io/conan-center-index/pull/12323, I need to check an option value of tinyspline in test() method of test_package, and it fails with:

```
ERROR: tinyspline/0.3.0 (test package): Error in test() method, line 31
if self.dependencies["tinyspline"].options.cxx:
AttributeError: 'NoneType' object has no attribute 'dependencies'
```

https://c3i.jfrog.io/c3i/misc/logs/pr/12323/4-linux-gcc/tinyspline/0.3.0//ab460bcf84a19ada9f4e0bfddca5324c1490a3ea-test.txt

self.dependencies should be exposed at least in `validate()`, `validate_build()`, `generate()`, `build()`, `package()`, `package_info()`, `test()`.