* 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)
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