Move dialyzer plt files around

They aren't huge, but when publishing mix just wanted to include
them. I could use the `files` option to exclude them but then
manually maintain that list and... I don't want to.

Having them in a separate folder seems better anyhow.

.github/workflows/main.yml

@@ -51,7 +51,7 @@ jobs:
       - name: Restore plts
         uses: actions/cache@v2
         with:
-          path: priv/plts
+          path: tools/plts
           key: ${{ runner.os }}-dialyzer-${{ matrix.elixir_version }}-${{ matrix.otp_version }}-${{ hashFiles(format('{0}{1}', github.workspace, '/mix.lock')) }}
       - run: mix deps.get
       - run: MIX_ENV=test mix compile --warnings-as-errors

.gitignore

@@ -28,5 +28,5 @@ benchee-*.tar
 # Misc.
 save.benchee
 /test/tmp/
-/priv/plts/benchee.plt
-/priv/plts/benchee.plt.hash
+/tools/plts/benchee.plt
+/tools/plts/benchee.plt.hash

mix.exs

@@ -27,7 +27,7 @@ defmodule Benchee.Mixfile do
       ],
       dialyzer: [
         flags: [:unmatched_returns, :error_handling, :race_conditions, :underspecs],
-        plt_file: {:no_warn, "priv/plts/benchee.plt"}
+        plt_file: {:no_warn, "tools/plts/benchee.plt"}
       ],
       name: "Benchee",
       description: """