Improve naming of different lib types

[jbrunton]

Currently we have:

• files and directories added as "libs" (i.e. added to template engine load path)
• .gflowslib packages
• the main context package

Option A:

• Consider a lib to be any dependency (file, dir or package) added to the build path
• Consider a package to be a special form of directory which includes workflows/ and libs/ directories.
• Rename .gflowslib to .gflowspkg?

Option B:

• Rename the libs config option to paths
• A library then refers only to a .gflowslib package.

Option C:

• distinguish between packages and libs (or paths) in config.
• could link to root folder instead of manifest.

templates:
  engine: ytt
  libs:
  - vendor
  dependencies:
  - https://example.com/my-lib

Also consider: standardize package manifests on gflowspkg.json (or gflowslib.json, or gflows.pkg.json).

Pros of A: naming consistent with packages in NPM ecosystem and others. Pros of C: simplify code and intent.