* Fix reindex with uninstalled deps
When a prefix of a dep is removed, and the db is reindexed, it is added
through the dependent, but until now it incorrectly listed the spec as
'installed'.
There was also some questionable behavior in the db when the same spec
was added multiple times, it would always be marked installed.
* Always reserve path
* Only add installed spec's prefixes to install prefixes set
* Improve warning, and ensure ensure only ensures
* test: reindex with every file system remnant removed except for the old index; it should give a database with nothing installed, including records with installed==False,external==False,ref_count==0,explicit=True, and these should be removable from the database
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