Add better handling for 'app not found'

[tusharsadhwani]

• I have added an entry to docs/changelog.md

Summary of changes

Added one feature:

• If the package is not a runnable app, and there is only one runnable app in the package, run it with a note.

Improved the error message thrown when the package is not a runnable app, to include the command required to run the app.

Resolves #725

[gaborbernat]

Needs a test plus changelog, plus what the other reviewer requested.

[tusharsadhwani]

@gaborbernat I wasn't able to get most of the tests to pass on my machine even on master branch, so I can't reliably add a test here. Plus, I'm not entirely how to test this fix as well.

[tusharsadhwani]

I can, however, provide a pretty good example of what was broken and is now fixed:

$ pipx run textflip hello            
'textflip' executable script not found in package 'textflip'. Available
executable scripts: flip

$  . venv/bin/activate

$ pipx run textflip hello
oןןǝɥ

$ pipx run --spec=textflip something hi
'something' executable script not found in package 'textflip'.
Available executable scripts:
    flip - usage: 'pipx run --spec textflip flip [arguments?]'

As for a package that has multiple runnable apps but neither of them matching the name of the package, I could find no examples sadly.

[gaborbernat]

Fix merge conflicts and run tests again.