Use preserve_paths instead of CocoaPods private

[paulb777]

Standardize and simplify internal FirebaseCore imports. Note the fast-follow #5760 changes the preserve_path to source_files so that the headers show up in generated Xcode projects.

• Change all client library Core imports to import the new umbrella FirebaseCore/Sources/Private/FirebaseCoreInternal.h instead of any specific header files.
• __has_include test so old IID used by FIS tests can find the Core private headers since the old podspec doesn't know how to find repo-relative headers. Also necessary for the latest Core to work with other old Firebase 6.x pods.
• Bump FirebaseCore minor version and the FirebaseCore dependency version in all dependents.
• Fix localize_podfile script to include ABTesting as an inner pod.
• Update some sample code that was unnecessarily using private headers.
• Fix an RC/Core symbol collision on FIRErrorCodeConfigFailed
• Add #if __cplusplus for public header imports from the Core Firebase-internal headers since Objective C++ modules don't work for unqualified public header names. I changed this back to a #if SWIFT_PACKAGE check in Migrate from interop pods to source_files header access #5760 since other issues came up with public header references from private headers there - so this can mostly be disregarded.

The first commit is large, but purely the mechanical change of all of the import changes (from a small Swift script). The interesting changes are in the commits that follow.

I tested with the private_headers removed from the FirebaseCore podspec. I'm restored the private_headers before merging to avoid a breaking change before 7.0.

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[ryanwilson]

One minor request - it may be helpful for future readers to have a comment at the top of this file specifying why it exists and why it's necessary.

[paulb777]

Will add in #5760

[lizhaobomb]

I run into this error FirebaseCore/Sources/Private/FirebaseCoreInternal.h not find

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