New in version 2.5.
True when expr is a string and is underscore-delimited lowercase package name:
abjad> iotools.is_underscore_delimited_lowercase_package_name('foo.bar.blah_package')
True
False otherwise:
abjad> iotools.is_underscore_delimited_lowercase_package_name('foo.bar.BlahPackage')
False
Return boolean.