[AggregateXCConfig] Set EMBEDDED_CONTENT_CONTAINS_SWIFT when using a …

[Resolver] Give a meaningful message for the case where there is no stable version for a pod, and user do not explicitly specify the required version.

Previous message was not covering the case where there are only pre-release versions between two stable versions, and user specifies version requirements for the pod such that only possibilities satisfying version requirements are pre-release versions. New message format also includes version requirements set by user in order to give a more explanatory output.

Added `source` and valuable flags to Podfile template

Provides a meaningful error message for the dependency conflict occuring when there is no stable version available for a given pod and user do not specify pod version explicitly in Podfile.

While handling resolver error in `handle_resolver_error` method of `resolver.rb` file, we will check if the specificati
on that is not satisfying given requirement was a pre-release version, and if the requirement was including any explicit version or source. If spec is a pre-release version, and there is no explicit version or source for the requirement, we will print a message directing the user to specify pre-release version explicitly.

* commit '2c3185a1':
  [CHANGELOG] Spelling fixes
  [Changelog] Entry for #4219
  Remove SDKROOT relative search path

Remove SDKROOT relative search path

This was causing warnings with the iOS 9 SDK as this path doesn't even
exist anymore and seemingly wasn't needed since XCTest came along.

Fixes #4168

All Podfiles should specify a `source` and with the growing number of Swift applications, having the `use_frameworks!` flag available is also very helpful.

Also, including the `inhibit_all_warnings!` will usually save a google search, as well. Most people haven't memorized all of the common cocoapod flags.

[Sandbox::FileAccessor] Add C++ Sources (.i)

[Validator] build watchOS pods against watchossimulator for validation 🌈

This reverts commit c521ad1e.

Added the cocoapods-search plugin to CocoaPods

[Resolver] Allow platform detection to work for nested dependencies