pkg: add @pkg-source alias

[Alizter]

Motivation

Package sources are currently fetched as a side effect of building packages. There is no way to fetch every locked package source up front, which makes it difficult to populate Dune's shared cache during a network-enabled phase and then build without network access. This is also useful groundwork for #9983.

Fixes #15628.

Implementation

• Add a context-level @pkg-source alias alongside @pkg-install.
• Make the alias depend directly on every fetched package source directory target. Packages without fetched sources, including local directory sources, are skipped because they require no network access.
• Report the same lock-directory guidance as @pkg-install when package management is inactive.
• Defer expansion of package build rules until their target is requested. Without this, loading a source directory rule could eagerly build a transitive package while resolving executables for the package build action.
• Expose pkg-source through dune show aliases.

Tests

• Verify @pkg-source fails with a useful message without a lock directory.
• Verify it populates source directories for direct and transitive packages without building either package.
• Verify the packages are subsequently built by @pkg-install.
• Update alias enumeration expectations.

Validation performed:

• dune build @check
• dune fmt
• dune runtest test/blackbox-tests/test-cases/pkg/alias-pkg-install.t test/blackbox-tests/test-cases/describe/aliases.t