[fuchsia] Migrate cr_package_fuchsia() to use SDK-provided rules.

This is a reland of
https://chromium-review.googlesource.com/c/chromium/src/+/2134299
which had broken handling of dependencies on the CMX manifest fragments
of each package's "primary" component. Processing of the fragment to
generate the full CMX file is now broken out into a distinct build
target, on which the primary component target depends.

Some unused Python and CMX files, missed in the original migration CL,
are also removed.

Migrate the cr_package_fuchsia() rule to use the SDK-provided
fuchsia_component() and fuchsia_package() rules internally.

TBR=dpranke

Bug: 1050703, 1068782, fuchsia:49479
Change-Id: I217d9276f85602c4120e3ad0db6d64c47c691f40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2141094
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757471}

• build/config/fuchsia/build_cmx_from_fragment.py[Added - diff]
• build/config/fuchsia/elfinfo.py[Deleted - diff]
• build/config/fuchsia/generate_runner_scripts.gni[diff]
• build/config/fuchsia/package.gni[diff]
• build/config/fuchsia/prepare_package_inputs.py[Deleted - diff]
• build/config/fuchsia/symbol_archive.gni[diff]
• fuchsia/engine/chromium.cmx[Deleted - diff]
• fuchsia/runners/cast/cast_runner.cmx[diff]
• testing/test.gni[diff]