deft update: don't descend into git submodules

[cgay]

If a package is using packages AND submodules, it should have registry files for the submodule libraries commited to Git and therefore it shouldn't be necessary for Deft to descend into submodules. Doing so causes these duplication warnings:

WARNING: Library "command-line-parser-test-suite" has multiple .lid files for platform "x86_64-darwin".
  /Users/cgay/dylan/workspaces/opendylan/_packages/deft/0.12.0/src/ext/command-line-parser/tests/command-line-parser-test-suite.lid
  /Users/cgay/dylan/workspaces/opendylan/_packages/deft/0.12.0/src/ext/command-line-parser/ext/testworks/ext/command-line-parser/tests/command-line-parser-test-suite.lid
  /Users/cgay/dylan/workspaces/opendylan/_packages/command-line-parser/3.2.0/src/tests/command-line-parser-test-suite.lid
  /Users/cgay/dylan/workspaces/opendylan/_packages/command-line-parser/3.2.0/src/ext/testworks/ext/command-line-parser/tests/command-line-parser-test-suite.lid
  /Users/cgay/dylan/workspaces/opendylan/sources/lib/command-line-parser/tests/command-line-parser-test-suite.lid
  /Users/cgay/dylan/workspaces/opendylan/sources/app/deft/ext/command-line-parser/tests/command-line-parser-test-suite.lid
  /Users/cgay/dylan/workspaces/opendylan/sources/app/deft/ext/command-line-parser/ext/testworks/ext/command-line-parser/tests/command-line-parser-test-suite.lid
Registry will point to the first one, arbitrarily.

[cgay]

Deft documentation should explain how to deal with this.