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 .swiftmodules
  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: #167592

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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