pkg/lib: lock clean-up, and add `path->pkg'

Make the installed-package database lock reentrant, change some
functions to take the lock, and fix the documentation on when a
lock is expected to be taken outside of functions.

original commit: 10e53e3bf4

collects/compiler/commands/test.rkt

@@ -108,12 +108,9 @@
        [l
         (for-each do-test l)])]
     [packages?
-     (unless (for*/or ([scope (in-list '(installation user shared))])
-               (define pd
-                 (parameterize ([current-pkg-scope scope])
-                   (with-handlers ([exn:fail? (λ (x) #f)])
-                     (pkg-directory e))))
-               (and pd (do-test pd)))
+     (define pd (pkg-directory e))
+     (if pd
+         (do-test pd)
          (error 'test "Package ~e is not installed" e))]
     [else
      (do-test e)]))