[foreign-memaccess+abi] RFR: Fix visibility issues

[Per Minborg]

On Fri, 22 Sep 2023 12:08:00 GMT, Per Minborg <pminborg at openjdk.org> wrote:

> This PR proposes to fix two visibility issues where package private interfaces are exposed via another class' public metods.

Getting rid of warnings reduces the probability of missing more important warnings down the line. But if the consensus is to drop this PR, we could certainly do that.

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PR Comment: https://git.openjdk.org/panama-foreign/pull/891#issuecomment-1734922896