* Refactor: installPackage -> installPackageFromRegistry

* Refactor: factor out source-agnostic installation steps

* Unpack and cache uploaded zip and tgz files.

* Add basic archive verification and parse manifest.

* Catch error when zip archive is uploaded as gzip.

* Add API integration tests.

* Remove unnecessary use of "package key" concept.

* Add 'install_source' property to saved object epm-packages.

* Adjust tests.

* Add API integration test for manifest missing fields.

* Refactor loadArchive -> loadArchivePackage.

* Refactor caching of package archive content

* Get datasets and config templates from manifest files.

* Use file paths from archive instead of asset paths from registry.

* Correctly load registry packages into cache

* Use InstallablePackage instead of RegistryPackage where possible.

* Actually install uploaded package.

* Add missing field to saved objects in tests.

* Adjust unit test to pick buffer extractor.

* Adjust unit test.

* Fix and re-enable getAsset() test.

* Adjust integration tests.

* Make error message match test.

* Pick data_stream.dataset from manifest if set.

* dataset -> data_stream also in comments

* Remove unused variable.

* Use pkgToPkgKey() where appropriate.

* More dataset -> data stream renaming.

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>