Family VFD prevents use of certain filenames with virtual datasets

**Describe the bug**
When using the Family VFD, creating files with certain filenames as source files for a virtual dataset can lead to an internal conflict within HDF5.

Specifically, it appears that because the family VFD splits files into member files and appends a number to the original filename to get the member filename. A user's request to open a file with that name will then fail due to it already being open.

**Expected behavior**
It should be impossible - or at least very unlikely - for a user to run into a filename conflict against internally-generated Family VFD files.

**Additional context**
This bug can be observed by running `test_dcpl_layout_caching()` on develop on any commit after #5537. The changes to the layout behavior in #5537 seem unrelated to this problem - it's just the file handling in the new test case that cause problems. See also #5673