Chris Bracken 0ce6126c47 [et] Lookup output filesystem path, not label (flutter/engine#52248)
Sets BuildTarget.executable to the `root_out_dir`-relative path of the
executable (e.g. `displaylist_unittests`) instead of its label (e.g.
`//out/host_debug/displaylist_unittests`). This is required since, in
the lines following the output lookup, we assume it to be a path
relative to the build output directory.

Also breaks out functions for:
* `_runGnDesc`: returns the JSON output of `gn desc buildDir target`
* `_runGnOutputs`: returns the output files listed by `gn outputs
buildDir target`

Noticed while working on:
https://github.com/flutter/flutter/issues/147071

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2024-04-22 11:13:16 -07:00
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