don't depend on datachain from PATH to exec processes

[skshetry]

Also turns internal udf runner option into subcommands.

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[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.86%. Comparing base (aa8f352) to head (5a921d3).

Files	Patch %	Lines
src/datachain/utils.py	50.00%	1 Missing and 1 partial ⚠️
src/datachain/cli.py	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
+ Coverage   86.85%   86.86%   +0.01%     
==========================================
  Files          88       88              
  Lines        9378     9379       +1     
  Branches     1879     1879              
==========================================
+ Hits         8145     8147       +2     
+ Misses        900      899       -1     
  Partials      333      333              

Flag	Coverage Δ
datachain	86.80% <72.72%> (+0.01%)	⬆️

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[dtulga]

I think the executable PATH changes are fine by me, as long as they work on the Studio environment. And it also looks like Studio changes will be necessary, as the ClickHouse tests are failing with:

Local Worker Error: usage: datachain [-h] [-V] command ...
datachain: error: the following arguments are required: command

Which sounds like the Studio / ClickHouse code needs to be updated to use the new argument design (if that part of the PR is kept).