port lint: warn if missing 'revision' field

[gpanders (Gregory Anders)]

New ports are required to contain the revision field, but 'port lint' currently does not show a warning or an error if this field is missing. It would be easier to enforce all ports having a revision if 'port lint' failed when the revision field is missing.

[ryandesign (Ryan Carsten Schmidt)]

The revision field is recommended. It would be nice if port lint would warn if it is missing. However the way that port lint analyzes a portfile may not be smart enough to be able to implement this completely. It's easy enough to do in a portfile containing a single port, but many portfiles contain many ports, implemented as subports. When the subports are closely related, like many python versions of a module (py37-setuptools, py38-setuptools, etc.) where each module has the same version, one revision line for all subports is appropriate. But if some of the modules are at a different version, or if the subports are less closely related (like graphviz, graphviz-gui, and gvedit) separate revision lines in each subport are appropriate. port lint does not know where in each portfile each subport is defined. port lint just reads the portfile line by line.