Don't clone shallow (#9)

Let users omit warnings and notes entirely

Useful for CI, in order to silence non-critical messages.

Allow limiting warnings to those affecting changed code

This allows gradual introduction of linting, hiding warnings for untouched
lines since a given point.

Ignore mkdir && rsync problem on last line of script

There, it is not a problem, since a failing mkdir would still set the overall
exit code to nonzero.

Always show errors from overridden recipes, even if not explicitly given

Add support for replacement specs

This is aliBuild's new feature for interpreting
"prefer_system_replacement_specs" keys in alidist recipes.

Update homepage URL to migrated repo

Recommend using alibuild-generate-module

Fix handling of list indices in error tree

Fix handling of stdin

Stdin is always a text stream, as it doesn't get passed through
`argparse.FileType('rb')`. Handle it specially.

For stdin, we also don't know the original file name, so skip the
check that the file name matches the package name.