fix: resolve relative file: dependencies correctly with install-strategy=linked

[manzoorwanijk]

In continuation of our exploration of using install-strategy=linked in the Gutenberg monorepo, which powers the WordPress Block Editor, this is a follow up of the fixes from #8996.

When using install-strategy=linked, relative file: dependencies (e.g., file:./project2) fail with ENOENT because the path is resolved one level above the project root. For example, file:./project2 becomes file:../project2, causing npm to look for /Users/dan/Source/project2 instead of /Users/dan/Source/project1/project2.

In the isolated reifier, non-workspace file: dependencies are incorrectly classified as "external" dependencies. This routes them through the store extraction path (pacote.extract) instead of symlinking them directly like workspaces. A Link node's resolved value is relative to its node_modules/ location — that relative path becomes invalid when used from the store context.

This PR treats local file: dep targets found in fsChildren as local dependencies (alongside workspaces) so they get symlinked directly instead of extracted into the store:

• _makeIdealGraph: Skip nodes from root.external if they (or their Link target) are in idealTree.fsChildren.
• assignCommonProperties: Classify deps whose targets are in node.fsChildren as local dependencies instead of external.

References

Fixes #7549