Better return value to distinguish between cannot install module vs not installed

[DonJayamanne]

Today when Jupyter extension uses the installModule API the return value is InstallerResponse.Ignore when:

• Pip or similar installation channels are not available (cannot install)
• Not installed (Python runs the installation scripts & then after than checks if it was successfully installed).

Would be good to have the following two instead:

• InstallerResponse.CannotInstall
• InstallerResponse.NotInstalled

Related to #15933 & #15934

[karthiknadig]

This feature is moving out to the envs extension. It should be handled there.