portlist: unexport GetList

It's an internal implementation detail, and I plan to refactor it
for performance (garbage) reasons anyway, so start by hiding it.

Updates #5958

Change-Id: I2c0d1f743d3495c5f798d1d8afc364692cd9d290
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f