Allow RAMDirectory to copy any Directory [LUCENE-7366]

Uwe: "The FSDirectory passed to RAMDirectory in the ctor could be changed to Directory easily. The additional check for "not is a directory inode" is in my opinion lo longer needed, because listFiles should only return files."

Use case: For increasing the speed of some of my application tests, I want to re-use/copy a pre-populated RAMDirectory over and over.

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Migrated from [LUCENE-7366](https://issues.apache.org/jira/browse/LUCENE-7366) by Rob Audenaerde
Linked issues:
- #7303

Start by inspecting RAMDirectory's constructor and the Directory.listFiles behavior described in the issue, including the existing inode check. Confirm that copying from any Directory supports the stated pre-populated RAMDirectory use case, then run the relevant directory tests and add coverage showing the expected copy behavior.

Written by the indexing model from the issue text.