Reland "[Fuchsia] Generate more complete GN target names for FIDL packages."

This is a reland of 0ed2b6cabb34340a5093b894250bd5c8292ed625

Original change's description:
> [Fuchsia] Generate more complete GN target names for FIDL packages.
> 
> FIDL package names consist of multiple dot-delimited segments, to allow
> scoping of packages to avoid clashes. Our GN generator script was using
> only the final segment of each package's name, resulting in clashes,
> e.g. between fuchsia.auth and fuchsia.modular.auth, if both were
> present in the SDK.
> 
> We continue to generate targets under the //third_party/fuchsia-sdk/sdk
> namespace, but use the full name rather than only the final segment,
> e.g. custom.modular.auth would become custom_modular_auth.
> 
> We also special-case the top-level "fuchsia" namespace to be ignored,
> e.g. fuchsia.modular_auth becomes just modular_auth.
> 
> Bug: 910670
> Change-Id: I675d894c72b5d43b6d8b20fb026075573a5a3380
> Reviewed-on: https://chromium-review.googlesource.com/c/1372892
> Commit-Queue: Wez <wez@chromium.org>
> Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#615763}

Bug: 910670
Change-Id: I2b7c5d766f5ee7cfe2d31b10f8410075efd52288
Reviewed-on: https://chromium-review.googlesource.com/c/1373276
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#615950}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 3a4d2b2ccc6ee5bac0ef1ff2e765749ea4199b2f
1 file changed