refactor: make rockcraft-pack follow new call conventions

[bepri]

This PR moves towards the ideal project structure moving forward. It mirrors the conventions being added in canonical/starflow#155.

• Each tool's actions will be contained in <tool>/<action>
• Tests and dists live alongside their action
• Shared behaviors live in common/ (this will replace _common/ once the composite actions are gone)

[Copilot]

Pull request overview

This PR refactors the Rockcraft pack action to follow the new “<tool>/<action> + shared common/” conventions, introducing a common workspace package and moving shared builder/action logic there.

Changes:

• Introduces a new common/ workspace package that centralizes shared tooling (tools.ts), builder logic (craft-builder.ts), and a generic pack runner (pack-action.ts) with unit tests.
• Adds a new Node action at rockcraft/pack and updates the existing rockcraft-pack entry action to be a composite wrapper that checks out and calls rockcraft/pack.
• Converts the repo build/test scripts and workspace configuration to a pnpm monorepo layout.

Reviewed changes

Copilot reviewed 8 out of 17 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tsconfig.json	Updates TS project includes to focus on common/ and rockcraft/pack/ and excludes dist/.
rockcraft/pack/tests/rockcraft-pack.test.ts	Updates Rockcraft pack tests to use the new common package imports and new index entrypoint.
rockcraft/pack/src/index.ts	Switches Rockcraft pack implementation to reuse shared common builder/action helpers.
rockcraft/pack/package.json	Adds a workspace package definition and build/test scripts for the Rockcraft pack action.
rockcraft/pack/action.yaml	Adds the new Node action definition for rockcraft/pack.
rockcraft-pack/action.yml	Converts rockcraft-pack into a composite wrapper that checks out and calls rockcraft/pack, and wires outputs.
pnpm-workspace.yaml	Declares the workspace packages (common, rockcraft/pack).
pnpm-lock.yaml	Updates lockfile to include new workspace importers and dependencies.
package.json	Changes root scripts to run build/test recursively in the workspace.
common/tests/tools.test.ts	Adds unit tests for new shared tooling helpers in common/src/tools.ts.
common/tests/pack-action.test.ts	Adds unit tests for shared input parsing and pack runner behavior.
common/tests/craft-builder.test.ts	Adds unit tests for the shared builder base class behavior.
common/src/tools.ts	Adds shared environment/tooling helpers (snapd/LXD/tool install + networking adjustments).
common/src/pack-action.ts	Adds shared input parsing and a generic “run pack action” helper.
common/src/craft-builder.ts	Adds shared builder base class used by tool-specific builders (e.g., Rockcraft).
common/package.json	Adds the new @craft-actions/common workspace package with exports.

Files not reviewed (1)

• pnpm-lock.yaml: Generated file

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[alesancor1]

LGTM, it's great. Just a small note on the comment for rockcraft-pack, see below: