bugfix: spack shouldn't fail in an incomplete environment (#16473)

Fixed #15884.

Spack asks every package linked into an environment to tell us how
environment variables should be modified when a spack environment is
activated. As part of this, specs in an environment are symlinked into
the environment's view (see #13249), and the package calculates
environment modifications with *the default view as the prefix*.

All of this works nicely for pointing the user's environment at the view
*if* every package is successfully linked. Unfortunately, right now we
only track what specs "should" be in a view, not which specs actually
are. So we end up calculating environment modifications on things that
aren't linked into thee view, and the exception isn't caught, so lots of
spack commands end up failing.

This fixes the issue by ignoring and warning about specs where
calculating environment modifications fails. So we can still keep using
Spack even if the current environment is incomplete.

We should probably also just avoid computing env modifications *entirely*
for unlinked packages, but right now that is a slow operation (requires a
lot of YAML parsing). We should revisit that when we have some better
state management for views, but the fix adopted here will still be
necessary, as we want spack commands to be resilient to other types of
bugs in `setup_run_environment()` and friends. That code is in packages
and we have to assume it could be buggy when we call it outside of builds
(as it might fail more than just the build).

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