nix: composable overlays (#578)

## Tool Updates

* Updated nix-eda to `0814aa6`: more orthodox approach to managing
dependencies by overlaying them on top of nixpkgs, which fixes an
occasional "repeated allocation" issue and helps make override behavior
more consistent.
  * Yosys and first-party plugins updated to 0.46 
* OpenLane itself no longer included in `devShells.*.dev`,
`devShells.*.docs`
* These shells are intended to be actual dev shells, i.e. used to
develop OpenLane, and needing OpenLane to pass tests to run these shells
makes no sense.

.flake8

@@ -19,6 +19,7 @@ exclude =
     venv/,
     test/designs/*.py,
     examples,
-    sandbox/
+    sandbox/,
+    test/,
 per-file-ignores =
     */__init__.py:F401

default.nix

@@ -31,12 +31,7 @@
   verilog,
   volare,
   yosys,
-  yosys-synlig-sv,
-  yosys-lighter,
-  yosys-sby,
-  yosys-eqy,
-  yosys-ghdl,
-  yosys-f4pga-sdc,
+  yosysFull,
   # Python
   buildPythonPackage,
   click,
@@ -57,14 +52,10 @@
   ioplace-parser,
   poetry-core,
 }: let
-  yosys-env = yosys.withPlugins ([
-      yosys-sby
-      yosys-eqy
-      yosys-lighter
-      yosys-synlig-sv
-      yosys-f4pga-sdc
-    ]
-    ++ lib.optionals (builtins.elem system ["x86_64-linux" "x86_64-darwin"]) [yosys-ghdl]);
+  yosys-env = (yosys.withPythonPackages.override {target = yosysFull;}) (ps:
+    with ps; [
+      click
+    ]);
   openroad-env = openroad.withPythonPackages (ps:
     with ps; [
       click