Short-circuit crawling all targets in et {build|test|query}

[matanlurey]

Closes flutter/flutter#147666.

This is a large change, I'd be happy to either review synchronously or change commands one at a time, but I think this is overall the right approach. I also didn't see any reason to reuse the BuildRunner code for these commands, the flow is basically:

graph LR
    A[et targetsOrPatterns] --> B[gn desc --format=json]
    B --> C[existing code that runs ninja/workers]

Quick summary of changes:

• Introduced a Label and TargetPattern type to avoid awkward string parsing/manipulation all over
• Replaced gn_utils (i.e. targetsFromCommandLine and friends) with an invocation of gn desc
• Some tests were brittle in terms of expected output, I mostly left them alone and wrote TODOs where applicable

Here is the fun part, results (and some manual integration tests):

# build, previously was 10-15s, is now ~3s
flutter % time et build -c host_debug_unopt_arm64 //flutter/impeller:impeller_unittests
[2024-05-14T19:05:55.163][macos/host_debug_unopt_arm64: GN]: OK
[2024-05-14T19:05:56.119][macos/host_debug_unopt_arm64: RBE startup]: Proxy started successfully.
[macos/host_debug_unopt_arm64: ninja] 0.0% (0/1) Regenerating ninja files[2024-05-14T19:05:57.309][macos/host_debug_unopt_arm64: ninja]: OK
[2024-05-14T19:05:57.573][macos/host_debug_unopt_arm64: RBE shutdown]: Actions completed: 0
$ENGINE/flutter/bin/et build -c host_debug_unopt_arm64   3.20s user 0.87s system 107% cpu 3.776 total

# test, also benefits from speedup, but I didn't time it
flutter % et test -c host_debug_unopt_arm64 //flutter/fml:fml_unittests
[2024-05-14T19:07:01.843][macos/host_debug_unopt_arm64: GN]: OK
[2024-05-14T19:07:02.707][macos/host_debug_unopt_arm64: RBE startup]: Proxy started successfully.
[2024-05-14T19:07:07.400][macos/host_debug_unopt_arm64: ninja]: 100.0% (3/3) LINK ./fml_unittests
[2024-05-14T19:07:07.404][macos/host_debug_unopt_arm64: ninja]: OK
[2024-05-14T19:07:07.748][macos/host_debug_unopt_arm64: RBE shutdown]: Actions completed: 1 (1 racing local)
OKAY:         7s.95ms //flutter/fml:fml_unittests

# query, also benefits from speedup
flutter % time et query targets -c host_debug_unopt_arm64
//flutter/display_list:display_list_benchmarks
//flutter/display_list:display_list_builder_benchmarks
# ... many targets omitted ...
$ENGINE/flutter/bin/et query targets -c host_debug_unopt_arm64  1.27s user 0.18s system 147% cpu 0.978 total

In other words, ~5x improvement on et build and et query is much faster as well.