Write project.pbxproj in Xcode's layout

[erneestoc]

Depends on #3381, which makes pbxproj_prefix write the whole PBXProject.

The partials are concatenated with cat, so objects come in generator order without the /* Begin <isa> section */ blocks Xcode writes, and Xcode rewrites the whole file the next time it saves the project.

cat can't produce those: markers are only written for non-empty sections, and objects of the same isa come from several generators with interleaved identifiers. A small project_pbxproj generator replaces cat. It finds each object's bounds and isa, stable sorts the objects by (isa, identifier), and writes them with the section markers, copying each object as is (the generators already write attributes in Xcode's order). Malformed partials (no objects = { line, an object without an isa, or an unterminated object) are an error.

On a 102 MB project.pbxproj (median of 11 runs):

	Time
cat	0.02 s
project_pbxproj	0.07 s

That's about 0.3 s for a 417 MB project. The action is still no-cache/no-remote/no-sandbox with the same inputs, so only this step reruns on regeneration.

The output matches Xcode byte for byte: a test project generated with this on top of the synchronized folders PRs (#3376–#3378) is identical to the file Xcode saved for it, and rules_xcodeproj's own project is unchanged from the previous version of this PR.

[brentleyjones]

Before even looking at the PR, I have concerns. We chose the cat model specifically because a 0.16 s per MB speed is too slow. Our project is 417MB. We can't have the last part of project generation take 66s.

[erneestoc]

Agreed, the script was far too slow. I've reworked it:

• Write the whole PBXProject in pbxproj_prefix #3381 makes pbxproj_prefix write the whole PBXProject in Xcode's order. It's the only object that was split across partials, so nothing else needs reordering.
• This PR is now a Swift generator that only finds object bounds, sorts them by (isa, identifier), and writes them with the section markers. Objects are copied as is. Its own commit is about 965 lines: about 265 of logic, 255 of tests, and the rest the usual generator setup (BUILD files, README, wiring). Until Write the whole PBXProject in pbxproj_prefix #3381 merges, the diff here also includes that PR's commit.

On a 102 MB project.pbxproj, it takes 0.07 s (cat takes 0.02 s). That's about 0.3 s for your 417 MB project. The output is byte-identical to the previous version and to an Xcode-saved project.