Implement installation of cookbooks from lockfile

Rename spec for policyfile lock building

Create policyfile lock objects from native types

Add `name` to policyfile DSL

Emit policyfile lock JSON in test script

Rename testing script

Pass policyfile name from DSL to lockfile

WIP - install cookbooks from lock data

Bump dependency on cookbook-omnifetch

Newer version supports exporting lock data as a hash instead of
berksfile.lock formatted string.

Update tests to expect source_options in policyfile lock

Test installation from cookbook lock objects

Test delegation methods for lockfile cookbook installation
Extract storage configuration to a class
Rename #to_source_options to #source_options_for_lock
Make population of cookbook information from local disk explicit
Document which methods are required by Omnifetch's contracts
Rename CookbookSpec => CookbookLocationSpecification
Extract cookbook lock objects to their own file
Use cookbook location specs to install from lockfile
Add tests for cookbook locks components
Extract superclass for cookbook locks
Tone down TODOs for some code duplication
Merge branch 'install-from-lock'

Adds library code that reads a Policyfile.lock.json and downloads cookbooks to
the cache.