Merges iOS and macOS configuration and SDK finder scripts:

* //build/mac/find_sdk.py and //build/ios/config/ios_sdk.py merge to
  //build/mac/apple_sdk.py.

* //build/config/ios/ios_sdk.gni and //build/config/mac/mac_sdk.gni
  merge to //build/config/apple/apple_sdk.gni. gn args for macOS are
  available on all builds with `host_os == "mac"`. gn args for iOS are
  available on builds where `is_ios` is true (in place of the existing
  conditional import logic). Merging the two allows us to check all
  Apple SDK-related variable for anything missing and run `apple_sdk.py`
  a maximum of once across all Apple platforms.

* Adds `Platforms` directory to `//flutter/prebuilts`, which contains
  Apple platform symlinks for `iPhoneOS`, `iPhoneSimulator`, `MacOSX`.
  Adds `ios_device_platform_path`, `ios_simulator_platform_path`,
  `mac_platform_path` gn args to hold these paths. Adds logic to
  `//flutter/tools/gn` to locate these paths and prepopulate these args
  from them to avoid running the script as part of the build.

  These are used to link to Swift core libs for each platform where
  necessary, as well as (for tests) test frameworks and `.swiftmodule`s
  like XCTest.framework.

* Eliminates `Library/Frameworks` symlink from `//flutter/prebuilts`.
  This path is a subdirectory of each of the platform directories above.
  It was used to populate `platform_frameworks_path` in
  `//flutter/shell/platform/darwin/ios/BUILD.gn` using sort of sketchy
  relative-path logic.

  Further, the existing logic for greating the link was buggy for runs
  of `ios_sdk.py` where the `--sdk` option was not specified (i.e. all
  runs where `--as-gclient-hook` were specified). Without this option,
  we cycle through both `iphoneos` and `iphonesimulator` SDKs, and since
  this symlink was written on each iteration through the `SDKs` list, it
  was always linked to the `Library/Frameworks` directory for the last
  item in the list: `iPhoneSimulator.platform`. The platform arg for the
  current build is now resolved to `ios_platform_path` from which we
  resolve `$ios_platform_path/Developer/Library/Frameworks`.

A follow-up patch will refactor the iOS build file to extract an iOS
test config that eliminates some of the unnecessary duplication of
cc_flags, swiftflags, and ld_flags across targets.

Issue: flutter#167592

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we expect every widget to implement].
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above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
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Fixes] where supported.
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