Fix aws-c-cal-config.cmake

[graebm]

Issue:
If I installed aws-c-cal (along with aws-lc and aws-c-common), and then built a simple CMake application that did find_package(aws-c-cal REQUIRED), the application would fail to configure with error message:

[cmake] CMake Error at /usr/share/cmake/Modules/CMakeFindDependencyMacro.cmake:47 (find_package):
[cmake]   Found package configuration file:
[cmake] 
[cmake]     /home/ec2-user/install/lib64/aws-c-cal/cmake/aws-c-cal-config.cmake
[cmake] 
[cmake]   but it set aws-c-cal_FOUND to FALSE so package "aws-c-cal" is considered to
[cmake]   be NOT FOUND.  Reason given by package:
[cmake] 
[cmake]   The following imported targets are referenced, but are missing: AWS::crypto

I could fix the application by explicitly depending on crypto, like:

find_package(crypto REQUIRED)
find_package(aws-c-cal REQUIRED)

but that shouldn't be necessary. The application should only need to depend on aws-c-cal, and crypto should be pulled in transitively.

Analysis:
I sprinkled a bunch of print statements around. The failure occurred when aws-c-cal-config.cmake included aws-c-cal-targets.cmake. CMake would notice that "AWS::crypto" was required, but not yet defined, and so mark aws-c-cal for failure. Later, find_dependency(crypto) would get run and it would find "AWS::Crypto", but it was too late, aws-c-cal was already marked for failure.

Apparently, you must call find_dependency(crypto) BEFORE calling include(aws-c-cal-targets.cmake).

Description of changes:

• Move find_dependency() calls before include(aws-c-cal-targets.cmake)
• Use @VAR@ style variables in the config file, so that it's simpler for the installed config file to remember how it was originally built.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.