[Refactor] Use existing method as shortcut

Instead of re-implementing the same call chain
[Refactor] Renamed TargetInstaller#target to native_target
[Refactor] Renamed Target#target to native_target

This should make it more intuitive why we have to navigate there.
[Refactor] Renamed local var in spec
[Refactor] Renamed TargetInstaller#library to target
[Refactor] Extracted Generator::Header from PrefixHeader

Fixed wrong documented types drive-by.
Minor change for edge case, more intuitive empty line generating
[Doc] Fix attribute documentation

Thanks to @fabiopelosin.
[Changelog] Add entry for #2461
Merge pull request #2461 from mrackwitz/refactorings

Refactorings in Preparation to Framework Support
Various doc and style fixes.
[Sandbox::FileAccessor] Use constants instead of methods and fix header pattern.

Previously the header pattern was basically doing: `*.{.h}`, which has a dot too much,
but just happened to work because we don't actually use `Dir.glob` but implement our
own, which apparently isn't 100% similar.
Use the `PROFILE=filename` env var to profile work.

Currently supported printer types are:
* `filename.html`: GraphHtmlPrinter
* `filename.txt`: GraphPrinter

For more printers see
https://github.com/ruby-prof/ruby-prof/blob/master/README.rdoc#printers.
[Validator] Update validator for new Linter::Results class

Updates for https://github.com/CocoaPods/Core/pull/131.
[bundle] Update
[SourcesManager] Do not try to clone spec-repos in `/`.

Fixes #2723.
[Gemspec] Require Resolver
[Resolver] Start using the resolver gem!
[Resolver] Add rudimentary lockfile handling, equivalent to the old behavior
[ResolverSpec] Disable spec that really belongs in the Analyzer