spack chain (#8772)

* initial work to make use of an 'upstream' spack installation: this uses the DB of the upstream installation to check if a package is installed

* need to query upstream dbs when adding new record to local db

* prevent reindexing upstream DBs

* set prefix on specs read from DB based on path stored in install record

* check that Spack does not install packages that are recorded as installed in an upstream db

* externals do not add their path to install records - need to use 'external_path' to get path of upstream externals

* views need to check for upstream installations when linking metadata

* package and spec now calculate upstream installation properties on-demand themselves rather than depending on concretization to set these properties up-front. The added tests for upstream installations don't work with this new strategy so they need to be updated

* only refresh modules for local specs (not those in upstream packages); optionally generate local module files for packages installed upstream

* when a user tries to locate a module file for a package installed upstream, tell them to use the upstream spack instance to locate it

* support recursive upstream databases (allow upstream databases to use their own upstream databases)

* separate upstream config into separate file with its own schema; each entry now also includes a name

* metadata_dir is no longer customizable on a per-instance basis for YamlDirectoryLayout

* treat metadata_dir as an instance variable but dont set it from kwargs; this follows several other hardcoded variables which must be consistent between upstream and downstream DBs. Also update DirectoryLayout.metadata_path to work entirely with Spec.prefix, since Spec.prefix is set from the DB when available (so metadata_path was duplicating that logic)