feat(readTree): expose ability to invoke with rootDir=false

readTree gives special treatment to the directory on which it is
invoked -- for example, it won't read *.nix files in that directory.

This commit adds the ability to disable this special treatment, which
remains the default behavior.

Example use case:

  10029d3682

Change-Id: I306bea95f4d556f7090f3255e6da6bb410adbf57
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12939
Tested-by: BuildkiteCI
Autosubmit: Adam Joseph <adam@westernsemico.com>
Reviewed-by: tazjin <tazjin@tvl.su>

nix/readTree/default.nix

@@ -259,12 +259,12 @@ in
     , args
     , filter ? (_parts: x: x)
     , scopedArgs ? { }
+    , rootDir ? true
     }:
     readTree {
-      inherit args scopedArgs;
+      inherit args scopedArgs rootDir;
       argsFilter = filter;
       initPath = path;
-      rootDir = true;
       parts = [ ];
     };