Introduce a cache system

Copying & patching all the DSOs is a time consuming process (~10s on a
slow hard drive computer). We definitely don't want to go through it
for each process start, we need to introduce a cache.

For this cache, we go the concervative way. We're going to "resolve" a
DSO name (ie. find the DSO absolute path) and sha256-hash each DSO.
We're then going to compare the fingerprints to determine whether or
not we need to nuke and rebuild the DSO cache.

The cache state is persisted through a JSON file saved in the cache dir.

.envrc

@@ -0,0 +1 @@
+use nix

default.nix

@@ -19,12 +19,15 @@ pkgs.stdenvNoCC.mkDerivation {
     patchShebangs $out/bin/nixglhost
   '';
 
-  postCheck = ''
-    black --check $out/bin/nixglhost
+  doCheck = true;
+
+  checkPhase = ''
+    black --check src/*.py
     nixpkgs-fmt --check *.nix
+    python src/nixglhost_wrapper_test.py
   '';
 
   installPhase = ''
-    install -D -m0755 nixglhost-wrapper.py $out/bin/nixglhost
+    install -D -m0755 src/nixglhost_wrapper.py $out/bin/nixglhost
   '';
 }