A plugin developed inside an application replaces its required plugins
with sibling directories (../user). Checked out on its own, that
directory is missing and plugin.yaml lost the minimum (golem15.user: ""),
so the manifest differed between the two layouts and TestPluginManifest
could not pass in both. Such a module is now fetched at the version
go.mod requires, as a consumer would see it.
- listedModule.fallback marks a local replace whose directory is absent
- fallback modules download separately, so one unfetchable version does
not hold up the others
- TestDeriveMissingReplace over a file:// proxy (offline)
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