Place repositories (git/oci) behind the Repository interface and use a Factory to create instances of implementations

[liamfallon]

This PR refactors the existing Porch code to make it easier to introduce a database cache.

It reinforces the structure of the code so that repository implementations (git/oci) are behind the interface and are only used by calling the interface. The repoimpl package is introduced as a package that contains "final" repository implementations. The following list details the refactors:

• A new pkg/repoimpl package is introduced, this package contains a factory for creating implementations of the Repository type that store packages that are source or target packages for Porch (such as git and oci), that is Repository implementations that are not Porch internal caches
• The pkg/git' package is moved to pkg/repoimpl/git`
• The pkg/oci' package is moved to pkg/repoimpl/oci`
• Types for the repoimpl factory are in pkg/repoimpl/types (to avoid circular imports)
• The RepoImplFactory interface is introduced to create instances of repos (git or oci today)
• The CreateRepoImpl function in pkg/repoimpl/repoimpl.go creates an instance of a repo based on the incoming configuration, calling the factory in either git or oci
• The cache options are restructured with a contained RepoImplOptions struct containing the options that the RepoImpl implementations use but the cache does not use
• A CacheFactory interface is introduced for creating cache implementations, where the implementation of the cache is behind the factory interlace
• A CacheFactory implementation is added to the memory cache.

[nephio-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: liamfallon

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[liamfallon]

/assign @kispaljr @efiacor @kushnaidu @JamesMcDermott @Catalin-Stratulat-Ericsson

[efiacor]

Few minor comments.
Do we need some unit test coverage on the new repoimpl stuff? Not sure if it's covered currently.

[liamfallon]

Few minor comments. Do we need some unit test coverage on the new repoimpl stuff? Not sure if it's covered currently.

yes, I'll do the unit tests in a later PR.

[nephio-prow]

@liamfallon: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
presubmit-nephio-go-test	87268b9	link	true	/test presubmit-nephio-go-test

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