Support for workflows within libraries

[jbrunton]

TODOs:

• Enforce directory structure in libs
• e2e test for jsonnet lib with workflow
• Implement libs with workflows for ytt
• e2e test for ytt lib with workflow
• rename "libs" -> "files" in lib manifest
• reinstate tests for local libs (not gflowslib packages)
• consider better names for different types of packages/libs (especially local gflowslibs vs libs directories) (Improve naming of different lib types #160)
• fix TODO in ytt engine getWorkflowSourcesInDir()
• define and validate behavior for observable workflow sources and templates for jsonnet engine
• define and validate behavior for observable workflow sources and templates for ytt engine
• update docs (Update docs for remote workflows in .gflowslib packages #162)