Pkgcore

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- pkgcore.ebuild.repository: Rewrite package manifesting support to allow
partial manifest writes via pkgdev.

- pkgcore.ebuild.portage_conf: Drop PORTAGE_CONFIGROOT and DISTDIR env support.

- pkgcore.ebuild.portage_conf: Handle finding the config directory for prefix
installs.

- pkgcore.ebuild.portage_conf: Only register existent repos for the config, but
they're still registered as raw repo_config objects so they can be synced as
wanted.

- pkgcore.config: Drop unused append_sources param for load_config().

- pkgcore.util.commandline: Drop --new-config/--add-config options.

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- pkgcore.pytest: Add support for keeping arches file updated.

- pkgcore.ebuild.domain: Don't configure external repos by default.

- pkgcore.ebuild.repository: Prefer using external repos over configured ones
for matching repo IDs.

- Use tarball syncer for the gentoo repo by default and drop sqfs entries.

- pkgcore.ebuild.repo_objs: Add 'external' attribute to signify an unconfigured
repo.

- pmaint: Drop perl-rebuild subcommand.

- pkgcore.ebuild.repo_objs: Support proxied attribute for maintainers.

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- pkgcore.ebuild.repo_objs: Add support for parsing upstreams from
metadata.xml.

- pkgcore.ebuild.repo_objs: Support pulling sign-commits setting from
metadata/layout.conf.

- pkgcore.ebuild.repository: Support creating repos with custom classes.

- pkgcore.util.commandline: Drop deprecated main() wrapper.

- pkgcore.pytest: Use full version for ebuild file creation.

- pmaint: Drop ``pmaint digest`` support in favor of ``pkgdev manifest``.

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- pkgcore.pytest: Add initial pytest plugin for ebuild and git repo fixture
support.

- pkgcore.util.commandline: Add support for suppressing help output for highly
pkgcore-specific options users generally shouldn't touch but are needed
internally.

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- Default to tarball-based syncing instead of using sqfs archives in
fallback config to avoid requiring elevated permissions for CI
actions.

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- Use user cache directory for repo storage when not running on a Gentoo
system.

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