Composer packages, autoloading non-class based files

When I was digging into the source of a Composer package on github I noticed that there were php files that matched namespace names but were preceded with an underscore. Puzzled I pulled the package down (via Composer) and noticed that the class loader that Composer generates required these underscored files explicitly, not autoloading as I'd expected.

For instance, in the crunch/regular-expression package there is a namespace called Crunch\RegularExpression:

-- src
---- Crunch
------- RegularExpression       <-- folder containing classes
------- _RegularExpression.php  <-- file namespace to Crunch/RegularExpression
                                    containing functions and constants 
                                    (instead of a class)

Initially I thought these underscored files were a feature of PSR-0 that I had missed, but then I looked at the Composer generated autoload_real.php and saw that _RegularExpression.php (amongst others) was being required explicitly:

…
$loader->register(true);

require $baseDir . '/src/Crunch/_RegularExpression.php';
require $baseDir . '/src/Crunch/RegularExpression/_Modifier.php';
require $baseDir . '/src/Crunch/RegularExpression/Pattern/_Modifier.php';
require $baseDir . '/src/Crunch/RegularExpression/Pattern/_Assertion.php';

return $loader;
…

Haven't been able to find any meaningful documentation about this feature of Composer. Is it a good "standard" for exporting non-class based namespaced dependencies, like functions and constants?

Update

My question turned out to be a slight misnomer. The selected answer lead me to discover that non-class based assets can be explicitly declared for loading in composer.json:

"autoload": {
    &q