Refactor file system source and path

[ilongin]

Before, when listing local FS, we had root of the FS always set for source field, e.g file:/// and the rest was in path. Idea behind this was to utilize partial indexing and remove obsolete add_storage method.
Now, when we are moving away from partial and bucket tables and moving indexing / listing to application level, we don't need this kind of setup as it was not convenient or intuitive.

With this changes when someone does DataStorage.from_storage("file:///home/ivan/animals/") we will have something like this in the listing table:

source	path
file:///home/ivan/animals	cats.jpg
file:///home/ivan/animals	dogs/dog.jpg

This also adds re-indexing check in .from_storage() to avoid re-indexing if not needed.
Old code related to that root of the FS mentioned at beginning of description was removed.

Note that some tests were skipped as it was not possible to refactor them without diving deep into other important task that will be worked on soon as well: #318

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.36%. Comparing base (dfd7fb4) to head (0864e6e).
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@@            Coverage Diff             @@
##             main     #496      +/-   ##
==========================================
- Coverage   87.49%   87.36%   -0.14%     
==========================================
  Files          97       97              
  Lines       10122    10136      +14     
  Branches     1382     1386       +4     
==========================================
- Hits         8856     8855       -1     
- Misses        909      923      +14     
- Partials      357      358       +1     

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[rlamy]

LGTM. Removing so many special cases for cloud_type == "file" in the tests is nice!