amplify-xcode: foundation and import-config command (#950)
* amplify CLI iOS core structure
* tooling and general setup
* import-config command, CLI interface
* docs
* commit xcodeproj files and settings
* resolve home directory in base path, AmplifyEnvironment glob function
* clean AmplifyCommandEnvironment, simplify task execution
* amplifyFileManager abstraction, refactor AmplifyEnvironmentFileManager
* commandEnvironment unit tests
* unit tests CommandImportConfig
* test ImportConfigTasks
* remove currentDirectory property
* remove different types of task executor, add more unit tests
* remove date from copyright headers
* update copyright header
* test CommandEnvironment::contentsOfDirectory
* update inline docs, typealias task successful result
* check for config files
* set multiple targets, separate core from CLI, improve testability
* rename AmplifyCLI -> AmplifyXcode
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