All of my Purchased Music Files Gone From My Hard Drive

It has always been very basic to always maintain a backup copy of your computer. Just restore everything from your backup.

If you really want to do things that way you have to disable iTunes' "copy files to folder" and "organize" preferences settings and take on media organization completely yourself. This has additional repercussions such as making it difficult to relocate media to another drive, and certain of iTunes features such as automatic repairing of broken links will no longer work. I also don't know how it will affect things such as Apple Music service and utilizing the iCloud interface. Somebody doing this is probably at an advanced enough computer level so they also know how to routinely make backups of their entire system because you are fighting iTunes's way of doing things. You also need to know exactly how iTunes operates and even then it will do things you do not always understand why. As iTunes becomes more and more complicated so too is it easier to break some process and result in the unexpected.