performance: speed up `spack find` in environments
performance: speed up `spack find` in environments

`Environment.added_specs()` has a loop around calls to
`Package.installed()`, which can result in repeated DB queries.  Optimize
this with a read transaction in `Environment`.

LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack