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This is a prerequisite step toward supporting multi-package
deployments.

* Replaces all calls to fuchsia_executable_runner() with
package and script generation targets. The revised rules are
simple enough that fuchsia_executable_runner() is no longer a
major convenience.
* Delete test_runner_script() for the same reasons.
* Use sensible defaults for things like sandbox_policy to clean
up the parameters for most instances of the rules.

Bug: 	855242
Change-Id: I2f04e1baa9afaf24cfd5d979d33e9912a0475e55
Reviewed-on: https://chromium-review.googlesource.com/1110749Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569850}