nixvimInfo: access package.default using tryEval

[MattSturgeon]

Allow getting meta.homepage from the default package to fail gracefully when the nixpkgs channel lock is out-of-sync with ours.

This is a draft because we still need to:

• see if this fixes (or at least helps with) end-user mismatched/out-of-sync nixpkgs lock errors
• add a test to ensure plugin maintainers set a valid URL (or use a package that has meta.homepage)

[MattSturgeon]

Anyone have a reproducible example that would cause a "not found in pkgs" error without this PR?