[DO NOT MERGE] Migrates to Kotlin's upcoming SwiftPM import feature

[JayShortway]

Description

Migrates the library to make use of Kotlin's upcoming SwiftPM import feature. This Kotlin feature is in preview, and still has the following rough edges to smooth out:

Issue	Description	Affects
	Customers using direct integration (embedAndSignAppleFrameworkForXcode) will have to run integrateLinkagePackage to generate the _internal_linkage_SwiftPMImport package and commit it to version control.	consumers of purchases-kmp
KT-82766	Some Kotlin bindings are generated twice, once in the objcnames package. This shows unresolved type errors in the IDE, but it builds and runs fine.	developers of purchases-kmp
KT-74692	Hard to structure the project when multiple modules depend on the same SPM package. Cannot influence the namespace for the generated Kotlin bindings, nor are they exposed as if they were an api() dependency. Workaround: set kotlin.mpp.enableCInteropCommonization=true in gradle.properties.	developers of purchases-kmp
✅ Fixed in titan-208	integrateLinkagePackage creates a project.pbxproj file that fails validation by the checkSyntheticImportProjectIsCorrectlyIntegrated task which is run at build time. Can be silenced using the kotlin.kmp.suppressXcodeIntegrationCheck=true property.	developers of purchases-kmp

Note for review

I would highly recommend reviewing this 1 commit at a time. That reduces the complexity considerably. Most of the changed files are due to import statements. I also added a migration guide.

Release plan

My plan is to release this branch as 3.0.0-dev01 after approval, without merging it. Let me know what you think of that too.