installation: skip repository metadata for externals (#16954)

When Spack installs a package, it stores repository package.py files
for it and all of its dependencies - any package with a Spack metadata
directory in its installation prefix.

It turns out this was too broad: this ends up including external
packages installed by Spack (e.g. installed by another Spack instance).
Currently Spack doesn't store the namespace properly for such packages,
so even though the package file could be fetched from the external,
Spack is unable to locate it.

This commit avoids the issue by skipping any attempt to locate and copy
from the package repository of externals, regardless of whether they
have a Spack repo directory.