pw_package: Simplify chromium verifier download

Porting unittest requires to pull in much more source files. Listing
each file individually makes the list too long. As a trade-off, we'll
checkout some entire subfolders instead of picking individual ones
in them. Although a number of more files are downloaded, the sparse
checkout list is much shorter. Accordingly, we'll not generate a
sources.gni for gn build. The list of sources will be added in
third_party/chromium_verifier BUILD.gn.

Bug: 394
Change-Id: Id15b40cf933af6f6b5da1cc595c79f2650fbb01c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/49303
Reviewed-by: Rob Mohr <mohrr@google.com>
Commit-Queue: Yecheng Zhao <zyecheng@google.com>