replace targets with fetcher+reader system

[arlimus]

Fully removes the current target system, which typically worked this way:

target ---> resolve( ) --->  [ metadata, ruby_tests, ruby_libs ]

Although incredibly convenient, this system was convoluted and hard to understand, as it did 2 things at once: Fetch the target you are interested in and then understand it (as either an InSpec profile or flat Ruby files, though there was some early support for Serverspec folders).

The new system instead works like this:

#1: get raw contents; e.g. file, folder, zip, tar, url, ...
target ---> fetch( ) ---> Fetcher[target]
                          => files(), read(file)

# optional: find the root folder
Fetcher[target] ---> relative_target() --->  Fetcher[target@root_dir]

#2: understand contents; e.g. inspec, flat, scap, ...
Fetcher[target@root_dir] ---> read_source( ) --->  SourceReader[Fetcher[target]]
                                                   => metadata(), tests(), libraries()

What appears to be more complex, now fully exposes all steps and clearly defines them. It provides explicit access to many elements of that chain, that were so far hidden, and led to a lot of code-duplication (e.g. source-paths, especially when chaining fetchers; relative-to-root vs absolute paths; raw contents instead of just converted ruby code).

Mainly targets bugs with InSpec's json and check commands and anything that uses the profile class to work.

[chris-rock]

I like that we pass a profile to runners now. We may discuss if we change the direction here as well, but not as part of this PR.

profile = Inspec::Profile.for_target(detect_util, opts)
profile.run

[arlimus]

Good point, also a curious one. The advantage of passing multiple things to runner is that it can add all things to a run and then execute them. If we have that execution, would we need Profile.run anymore? I guess it's still great for convenience...

[srenatus]

@chris-rock LGTM.

[arlimus]

Awesome review, thank you @srenatus and @chris-rock !
+1