Speedup audbackend.interface.Maven.ls()

[hagenw]

In its current implementation, the execution time of audbackend.interface.Maven.ls() scales with the content of the repository / sub-folder, as it first collects all files given under root or a sub-folder, and limits the results only afterwards for the requested file.

As all versioned belonging to a file are stored in the the sub-folder <root>/<name>/ there is no need for this. In this pull request, I use ls() of the underlying backend object to inspect the content of that folder and hence speed up the execution time.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.0%. Comparing base (bdc5368) to head (8aa9d92).

Additional details and impacted files

| [Files](https://app.codecov.io/gh/audeering/audbackend/pull/226?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=audeering) | Coverage Δ | | |---|---|---| | [audbackend/core/interface/maven.py](https://app.codecov.io/gh/audeering/audbackend/pull/226?src=pr&el=tree&filepath=audbackend%2Fcore%2Finterface%2Fmaven.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=audeering#diff-YXVkYmFja2VuZC9jb3JlL2ludGVyZmFjZS9tYXZlbi5weQ==) | `100.0% <100.0%> (ø)` | |

[hagenw]

This is also the one advantage we have with the Maven interface, compared to the Versioned interface, as in Versioned we cannot limit the listing of files that easily.