Support for workflows within libraries

TODOs:

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

jbrunton / gflows