iterate over subdirectories of extra_src or add a flag to indicate adding source archive under a directory in its own layer.

Currently, if we have many source archives we want to add layer by layer to the source image the only way to do that is to put source archives in their own subdirectory and pass each subdirectory's absolute path with EXTRA_SOURCE_DIR.

But as @MartinBasti pointed out in https://github.com/containerbuildsystem/atomic-reactor/pull/1606#discussion_r609879731, if we do that then there's a good chance we may reach the limit length for a command in the shell.

Adding the ability to iterate over the subdirectories automatically and add them all layer by layer or adding a flag to indicate adding each source archive in a different layer will help solve this problem.

Start by tracing how EXTRA_SOURCE_DIR is parsed and how source archives are added to image layers. Compare iterating over its subdirectories with a flag that places each archive in a separate layer, while checking how absolute paths and shell command length are handled. Done means multiple archives can be layered without requiring one command-line argument per subdirectory.

Vom Indexierungsmodell aus dem Issue-Text verfasst.