branchcache: move head writing in a `_write_heads` method Same rational: this will help having format variants.

branchcache: move the header loading in a `_load_header` class method This will help changing header parsing in format variants.

branchcache: simplify a long line Gratuitous change to help code readability.

branchcache: rename `load` to `_load_heads` We are about to have more similar function, we rename the existing one to a more meaningful name and mark it private in the process.

branchcache: move the filename to a class attribute This prepare the introduction of more variant of cache.

test-clonebundles: simplify matching to be less flavor depends We keep the files and bytes output for the first call, but then we mostly check that we are being served a stream-clone bundle, not the actual content and size of the bundle. That aspect being tested by the stream clone test themselves.