Improve Xcode selection diagnostics

[erikkerber]

An attempt to (greatly) increase the legibility of the currently inscrutible error message developers receive when the configured Xcode is not available locally or remotely.

Bazel couldn’t match the selected Xcode to this project’s supported versions:
  27.0 (build 27A266a)

Supported versions:
  26.5 (build 17F42)
  27.2 (build 27B5019j)

Detected locally:
  26.5 (build 17F42)
  27.0 (build 27A266a)
  27.2 (build 27B5019j)

Install a supported Xcode version and select it with xcode-select.

Validation

• bazel test //test:xcode_config_test on Bazel 8.7.0, 9.0.0, 9.0.1, and 9.2.0: all 51 tests pass on each release, including unsupported, unknown, ambiguous, duplicate, and empty inventories, plus custom and empty hints.
• Verified the failure against a consuming project's configuration and checked the fallback when discovery fails.

A few examples:

[erikkerber]

Realizing this actually a terrible default because it's not the default way Xcode is configured.