Allow support for pods with xcasset bundles using `:path`.

Closes #1549
Closes #3384
Closes #3358

Closes #3344.

Closes #3345.

[Migrator] Update spec checksums in the manifest

Provide an actual path to save the script to

[AggregateTargetInstaller] Unique resources passed to the script generator

[Executable] Properly execute things using Open4.spawn using an array of...

[CopyResourcesScript] Do not discard `.xcassets` from the main project if there are any pods containing `.xcassets`.

[CopyResourcesScript] Do not discard `.xcassets` from the main project if there are any pods containing `.xcassets`.

Closes https://github.com/CocoaPods/CocoaPods/issues/3301.

[CoC] Adds a code of conduct, thanks for the inspiration @indirect

[PrefixHeader] De-duplicate the contents on the prefix_header file when there are multiple subspecs with the same prefix_header

Closes https://github.com/CocoaPods/CocoaPods/issues/3283.

[CHANGELOG] Used unicode for the emojis (as GitHub emoji codes does not seem to be rendered online) #2892