Fix layer path resolution

[GrahamCampbell]

Resolves layer package paths relative to serverless.serviceDir instead of process.cwd().

This cwd dependency was introduced in 3093c83 when layer packaging started pre-resolving layerObject.path with path.resolve(...) to support paths like ./layer and trailing slash variants. That made packaging sensitive to the shell cwd whenever cwd !== serviceDir.

The fix anchors the layer path once with serverless.serviceDir and uses that same path for both file resolution and ZIP prefix stripping.

Adds a regression test for packageLayer() with a different current working directory.

[Copilot]

Pull request overview

Fixes layer packaging path resolution so layer file paths are resolved relative to serverless.serviceDir (service root) rather than process.cwd(), and adds a regression test to ensure correctness when the current working directory differs from the service directory.

Changes:

• Resolve layerObject.path against serverless.serviceDir in packageLayer() and use that resolved path consistently for zipping inputs/prefix.
• Add a unit test that changes process.cwd() to validate packageLayer() still resolves layer files relative to serviceDir.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/plugins/package/lib/package-service.js	Adjusts packageLayer() to compute an absolute layerPath from serviceDir and use it for file path construction and zip prefixing.
test/unit/lib/plugins/package/lib/package-service.test.js	Adds regression coverage ensuring layer packaging is independent of process.cwd().

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