This commit implement scaning a package db directory to collect package
id
If you installed a package both in a sandbox and globally, global
package may be selected even if there's a package in a sandbox, which is
different behavior from cabal sandbox.
e.g. when you have fast-logger-2.0 globally and fast-logger-0.3.3 in a
sandbox:
(Without patch)
$ ghc-mod check Foundation.hs
Foundation.hs:12:31:Module `System.Log.FastLogger' does not export `Logger'
(With patch)
$ ghc-mod check Foundation.hs
46492a19b0