feat: better errors when unsupported packages are requested

[dom96]

• output is now captured for uv commands
• packages are now installed to venv first
• this enables us to determine the cause of installation errors more accurately
• error messages are now clear about the cause of the failure and let the user know if a package is unsupported by python workers

Test Plan

• with package version that works in native python but not in pyodide

$ uv tool install -e . # install dev copy of pywrangler
# in wrangler project with "dspy==3.0.3" in pyproject.toml
$ pywrangler sync 
INFO     Found 2 dependencies.                                                                                                                                                                                                               
INFO     Installing packages into .venv-workers...                                                                                                                                                                                           
INFO     Packages installed in .venv-workers.                                                                                                                                                                                                
INFO     Installing packages into python_modules...                                                                                                                                                                                          
WARNING  Using Python 3.13.2 environment at: .venv-workers/pyodide-venv                                                                                                                                                                      
           × No solution found when resolving dependencies:
....
ERROR    Installation failed because the packages you requested are not supported by Python Workers. See above for details.

• with package version that doesn't exist at all

# in wrangler project with "dspy==4.0.3" in pyproject.toml
$ pywrangler sync   
INFO     Found 2 dependencies.                                                                                                                                                                                                               
INFO     Installing packages into .venv-workers...                                                                                                                                                                                           
WARNING  Using Python 3.13.1 environment at: .venv-workers                                                                                                                                                                                   
           × No solution found when resolving dependencies:                                                                                                                                                                                  
           ╰─▶ Because there is no version of dspy==4.0.3 and you require dspy==4.0.3, we can conclude that your requirements are unsatisfiable.                                                                                             
ERROR    Failed to install the requirements defined in your pyproject.toml file. See above for details.