Change toolkit build folder from .aws-sam to .aws-toolkit

[zhangzhx]

Is your feature request related to a problem? Please describe.
Toolkit currently is using the same sam build folder to sam cli, .aws-sam. Toolkit should use a separate folder, say .aws-toolkit for the target build folder to avoid potential conflict issues.

This is blocked by sam cli issue since sam cli copies all the folder structure from the lambda base directory to the target build directory. If the lambda base directory is the same to the module root, it copies the toolkit build folder recursively to itself.

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