Build only the function I want to test

SAM CLI now supports building a single function instead of the whole template, as of [v0.16.0](https://github.com/awslabs/aws-sam-cli/releases/tag/v0.16.0). Currently the toolkit will run `sam build` when debugging and invoking the function. This will build the whole template and increase the time for a customer to debug/invoke. If instead, the toolkit passes the Functions LogicalID from the template to the build command `sam build MyFunction`, build will only build that function. This should only be used in the debug/invoke cases and the full template will still need to be built before package/deploy to ensure all functions are built before uploading to the cloud.

Start at the debug and invoke paths that currently run `sam build`, and trace how the function's logical ID is obtained from the template. Done means debug/invoke builds only the requested function, while package/deploy continues to build the full template; verify both paths with the existing test suite.

Written by the indexing model from the issue text.