[BUG] gracefully handle non-unique file names

[duncdrum]

Uploading multiple *.xar files with unique contents and versions, but non-unique filenames leads to overwrites.

e.g. my-app.xar cannot have multiple versions, without overwriting older ones.

Expected behavior
The name of the xar file should not have to be unique. the correct version is defined in expath-pkg.xml

Screenshots

Context (please always complete the following information):

• eXist-db Version: 6.2.0
• App Version: 4.1.1