Refactoring and debug target improvements

[vknabel]

This PR implements changes proposed in https://github.com/vknabel/vscode-swift-development-environment/issues/27#issuecomment-414717260.

First of all, it includes some major refactoring

• Introduces Current type
• Avoid uses of rootPath in favor of workspaceFolders for better workspace support
• Replaces allModulePaths and allModuleSources with targets
• Extracted different behaviors that wrote into allModulePaths and allModuleSources into Packages

Within this PR, a Package provide autocompletion Targets (module is a keyword in JS).

The following packages are known:

• availablePackages which includes all other packages. No new behavior.
• configPackage providing targets from swift.targets setting. New behavior.
• debugYamlPackage provides targets by reading the .build/(debug|release).yaml files. New behavior as proposed by https://github.com/vknabel/vscode-swift-development-environment/issues/27#issuecomment-414717260.
• description-package provides the old behavior of SDE as fallback if debugYamlPackage is not available using swift package describe --type json. No new behavior.
• swift-file-package provides a helper target containing all unused Swift files. No new behavior.

[vknabel]

Thanks for having a deeper look!

I plan to release the update this weekend. Until then the documentation needs an update (especially the FAQ)