Improve scraped definition when missing source package.

[jasonkarns]

Desired output:

perhaps a message that the source is missing, and only binaries are possible.

Somehow, still derive the package name so the install line is:

install_package "foo" (only missing the URL)

[jasonkarns]

Current definitions print using plain echo. Ought to leverage the same mechanism as eol and lts_maint warnings.