Subprojects support

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A project is at root level, in subdirectories there could be subprojects that have their own tasks.
There should be a possibility to import subprojects from subdirectories into a project.

Implementation details:

• To avoid magic, there should be explicite imports of subprojects
• Imported tasks should be prefixed ex. :subproject1 so a task could be :subproject1:task1
• Subproject task should be executed with a proper workdir set, to point to subproject directory
• :tasks task should include subprojects tasks
• Subproject-subprojects should be possible ex. :subproject1:docs:make

The functionality should work like in Gradle project.

Why, use cases:

• Building docs, infrastructure in single projects. Docs, infrastructure is something different than whole project
• There are projects that includes ex. server + client + mobile application. Each in separate directory, with its own build tasks

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Pipelines implementation details

Defining pipelines

:some-task :some-task2 should be for example translated to :dir1:dir2:dir3:some-task :dir1:dir2:dir3:some-task2 - assuming :dir1:dir2:dir3 is the project name due to directories inheritance.

Workdir should point to dir1/dir2/dir3

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Awaiting review.