Update CMakeLists.txt replace find_package with aws_use_package for aws-c-event-stream

[mattparks]

Issue #, if available: When building alongside aws-c-event-stream find_package pulls in the system library instead of the in source target.

Description of changes: This change brings over the concept of aws_use_package from aws-c-common to aws-cpp-sdk-core. I did not add any testing because with IN_SOURCE_BUILD=OFF the same fine_package behaviour is maintained.

Check all that applies:

• [x] Did a review by yourself.
• [ ] Added proper tests to cover this PR. (If tests are not applicable, explain.)
• [ ] Checked if this PR is a breaking (APIs have been changed) change.
• [x] Checked if this PR will not introduce cross-platform inconsistent behavior.
• [ ] Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

• [x] Linux
• [x] Windows
• [x] Android
• [ ] MacOS
• [ ] IOS
• [ ] Other Platforms

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[wps132230]

Hi there, thank you for your pull request. We are working with maintainers of aws-c-* to have a better approach to consume their libraries. And one improvement is to use aws_use_package.

[wps132230]

Hi @mattparks We've updated a lot in aws-cpp-sdk-core/CMakeLists.txt since 1.9.x. And it's using aws_use_package(aws-crt-cpp) instead of aws-c-event-stream now. Does it work in your scenario? And do you still need changes in this PR?

[jmklix]

https://github.com/aws/aws-sdk-cpp/issues/1888

[jmklix]

Closing this PR. Thanks for taking the time to make this PR, but it is now outdated thanks to some changes in the build process of this sdk.