fix(npm): correct package_store_prefix_len for links in external-repo sub-packages

[ma-oli]

_symlink_package_store strips package_store_prefix_len characters from a dependency's package_store_directory.short_path to form the relative symlink target. The length omitted the "/" separators that follow the package path and the repo name, so for a link target in a non-root package of an external repository it stripped two characters too few. The leftover was the trailing "s/" of ".aspect_rules_js/", producing dangling symlinks such as ../../s/<pkg>@<ver>/node_modules/<pkg> and ERR_MODULE_NOT_FOUND at runtime.

In a root module the miscount is only one character and is masked by "//" path collapsing, so it surfaces only when a module's npm packages live in a sub-package (e.g. //frontend) and that module is consumed as an external dependency.

Account for both separators: +1 after the package path and +1 after the repo name.

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Changes are visible to end-users: yes/no

• Searched for relevant documentation and updated as needed: yes/no
• Breaking change (forces users to change their own code or config): yes/no
• Suggested release notes appear below: yes/no

Test plan

• Covered by existing test cases
• New test cases added
• Manual testing; please provide instructions so we can reproduce:

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[jbedard]

Can you add a test reproducing the issue? I would think such an error would have been caught by now so there must be something unique going on here?