Cloudformation package command must resolve relative file paths when processing AWS::Include transform

[sanathkr]

Issue #, if available:
aws/aws-sam-cli#654

Description of changes:
aws cloudformation package has the behavior where it resolves relative file paths with respect to the folder where the template is present. But for the AWS::Include transform which was recently added (#3454), did not follow this pattern. As the result, it was resolving relative paths with respect to the os.cwd() which is where the AWS CLI is invoked from.

This PR fixes the issue. It also raises an explicit exception when the local file could not be read, so users get an immediate feedback loop when either their path is invalid or the file could not be found. This is also a pattern followed when packaging other resources.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-io]

Codecov Report

Merging #4083 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #4083      +/-   ##
===========================================
+ Coverage    94.46%   94.46%   +<.01%     
===========================================
  Files          187      187              
  Lines        14114    14118       +4     
===========================================
+ Hits         13333    13337       +4     
  Misses         781      781

Impacted Files	Coverage Δ
...customizations/cloudformation/artifact_exporter.py	98.57% <100%> (+0.02%)	⬆️

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