ArchiveWriter: support explicit entries and rewrite internal symlinks

[mazdak]

Context and Motivation

While building out a Docker Compose-style plugin and validating it against our own real development stack, we ran into two classes of problems in the build-context path, and to fix them we had to make changes to the container app, but part of the changes were archive-authoring concerns and belong in ContainerizationArchive, not in higher-level callers. The result is a reusable API for explicit archive creation and more portable archives when symlinks point to other archived content.

(Related container PR: apple/container#1391)

This PR adds a generic explicit-entry archive API to ContainerizationArchive and improves symlink handling so archives remain relocatable after extraction.

In addition to archiveDirectory(_:), callers can now provide an ordered list of source entries to archive. This makes it possible to archive a precomputed file set without rewalking the source tree and centralizes symlink handling in the archive layer itself.

Testing

• add regression coverage for explicit-entry archive creation
• add round-trip tests for:
  • internal absolute symlinks rewritten to archive-relative paths
  • external absolute symlinks preserved unchanged
  • canonicalized internal absolute symlinks through symlinked ancestors
• verify existing archive-directory symlink behavior still passes

Passed:

• swift test --filter archiveEntries
• swift test --filter archiveDirectoryRewritesInternalAbsoluteSymlink
• swift test --filter archiveDirectorySymlinkInside
• swift test --filter archiveDirectorySymlinkOutsideExcluded

[jglogan]

@mazdak Sorry I've not looked closely enough at the changes yet, I've been swamped.

I'll get some feedback on this by Friday. I still need to look at the code more closely to see where this best fits.