plugin: handle chunks of output correctly

[euanh]

Motivation

containertool produces its normal output in single writes of less than PIPE_BUF bytes, which should be delivered atomically through the pipe to the plugin:

Reading or writing pipe data is atomic if the size of data written is not greater than PIPE_BUF. This means that the data transfer seems to be an instantaneous unit, in that nothing else in the system can observe a state in which it is partially complete. Atomic I/O may not begin right away (it may need to wait for buffer space or for data), but once it does begin it finishes immediately.

https://www.gnu.org/software/libc/manual/html_node/Pipe-Atomicity.html

Progress messages written in this way are displayed correctly. Unfortunately error output from swift-argument-parser is delivered in smaller chunks which are often broken across several lines by the plugin:

% swift package --allow-network-connections all  build-container-image       
Building for debugging...
[1/1] Write swift-version-2380AA06D3543E1B.txt
Build of product 'containertool' complete! (1.74s)
Building for debugging...
[0/3] Write swift-version-2380AA06D3543E1B.txt
Build of product 'hello-world' complete! (2.04s)
[ContainerImageBuilder] itory <repository>'
Help:  --repository <
[ContainerImageBuilder] repository>  Repository path
[ContainerImageBuilder] Usage: containertoo
[ContainerImageBuilder] l [<optio
[ContainerImageBuilder] ns>] --re
[ContainerImageBuilder] posito
[ContainerImageBuilder] ry <rep
error: Missing expected argument '--repos
[ContainerImageBuilder] ository
[ContainerImageBuilder] > <ex
[ContainerImageBuilder] ecutable>
  Se
[ContainerImageBuilder] e 'containertool --help' for more inf
[ContainerImageBuilder] ormation
[ContainerImageBuilder] .

At one point the plugin reassembled output from the pipe and re-split it on newlines, but the code triggered strict concurrency warnings with the Swift 6 language mode so it was removed in favour of PIPE_BUF write atomicity. This commit restores the pipe output reassembly.

Modifications

• Plugin reassembles output from containertool and splits it on newlines
• All output after an error is now printed using Diagnostics.error. Otherwise error and progress output would be interleaved.

Result

Error output from containertool will be presented correctly by the plugin.

Test Plan

Existing tests continue to pass.
A new integration test checks that error output is not broken into multiple lines.