Load Gitiles web assets from source build

The Gerrit plugin currently extracts static assets from the Gitiles
servlet Maven artifact. Gerrit can only source-build Gitiles core from a
modules/gitiles submodule if the plugin reads those assets from Gitiles'
source-built webassets target instead.

Switch the resource genrule to
@gitiles//resources/com/google/gitiles:webassets and preserve the final
com/google/gitiles/+static layout expected by the plugin. This keeps the
plugin resource jar shape unchanged while removing its dependency on the
servlet artifact for static files.

This change only builds inside the Gerrit tree, once the companion
change adds the gitiles source submodule and the @gitiles repository;
depend on it so CI resolves @gitiles.

Depends-On: https://gerrit-review.googlesource.com/c/gerrit/+/623641
Depends-On: https://gerrit-review.googlesource.com/c/gitiles/+/623621
Depends-On: https://gerrit-review.googlesource.com/c/gitiles/+/623622
Depends-On: https://gerrit-review.googlesource.com/c/zuul/jobs/+/623741
Change-Id: Iacac0788e334520057f8e7eaa435fcb7be9b1cd5