Wrong file location when adding files to imported datasets

[mohammad-alisafaee]

Describe the bug
When adding a file to an imported datasets, the new file is added to a data directory with dataset's name instead of the dataset's directory.

To Reproduce

$ renku init
$ renku dataset import https://zenodo.org/record/3251128
$ ls data
contemporary_phenotypic_12
$ renku dataset add contemporary_phenotypic_12 https://example.com/index.htm
$ ls data
contemporary_phenotypic_12 'Contemporary phenotypic change in plant quantitative traits'

Expected behavior
Files must be added to the same folder.

[rokroskar]

related to #791