Change craftctl communication channel from named pipes to unix sockets to consolidate the ctl server and error stream capture. Process execution runs a selector to obtain data from i/o streams to obtain the isolated error output. Allow this selection loop run on a caller-specified selector with handlers registered to also process interactions with craftctl. The captured error output when executing scriptlets is passed to `ScriptletRunError`. Errors from `craftctl default` execution in the build step raise `PluginBuildError`. This is consistent with the exception raised by the built-in build step handler. Signed-off-by: Claudio Matsuoka <claudio.matsuoka@canonical.com> - [x] Have you signed the [CLA](http://www.ubuntu.com/legal/contributors/)? - [ ] Have you added an entry to the changelog (`docs/reference/changelog.rst`)? ----- **Notes to reviewers:** see individual commits for pipe-to-socket change, and socket selector consolidation.