feature #15719 Deprecate ResourceInterface::getResource() (mpdude)

This PR was merged into the 2.8 branch.

Discussion
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Deprecate ResourceInterface::getResource()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        | n/a

The return value of this method does not make sense if you do not exactly know about the type of resource at hand. For example, it may be [an array](https://github.com/symfony/symfony/blob/b49fa129bdb3c0aa970a006b16dd1ca63a9d7ebd/src/Symfony/Component/HttpKernel/Config/EnvParametersResource.php#L57) or a [file path](https://github.com/symfony/symfony/blob/87800ae47e64429f2544b798575d1cc1d4e5464a/src/Symfony/Component/Config/Resource/FileResource.php#L51).

As all usages of getResource() within Symfony are in tests of particular Resource implementations anyway, deprecating and later removing this method helps us with simplifying the ResourceInterface (#7176).

Commits
-------

87c0c7d Deprecate ResourceInterface::getResource()

src/Symfony/Component/Config/CHANGELOG.md

@@ -23,6 +23,10 @@ After: the code will work as expected and it will restrict the values of the
 
  * deprecated the `ResourceInterface::isFresh()` method. If you implement custom resource types and they
    can be validated that way, make them implement the new `SelfCheckingResourceInterface`.
+ * deprecated the getResource() method in ResourceInterface. You can still call this method
+   on concrete classes implementing the interface, but it does not make sense at the interface
+   level as you need to know about the particular type of resource at hand to understand the
+   semantics of the returned value.
 
 2.7.0
 -----

src/Symfony/Component/Config/Resource/ResourceInterface.php

@@ -47,6 +47,12 @@ public function isFresh($timestamp);
      * Returns the tied resource.
      *
      * @return mixed The resource
+     *
+     * @deprecated since 2.8, to be removed in 3.0. As there are many different kinds of resource,
+     *             a single getResource() method does not make sense at the interface level. You
+     *             can still call getResource() on implementing classes, probably after performing
+     *             a type check. If you know the concrete type of Resource at hand, the return value
+     *             of this method may make sense to you.
      */
     public function getResource();
 }