add check_for_undefined_functions option using xref on release

[tsloughter]

systools exref feature has no good way of setting the xref library path so I've added our own feature for this. A use can still enable exref if they desire but in docs we will focus on this new option.

It only adds project apps that are in the release (even if they are not listed directly but are a dependency of another project app listed in the relx release applications list) and sets the library path to only include applications in the release.

This is important for helping catch when a user forgets to include an application in the release that is used -- like forgetting to add a runtime dependency to .app.src .

Question I want considered before we merge this: Should it be on by default?

I'm learning towards making it on by default.

Future work that should be done:

• Option to include non-OTP dependencies in the analysis. Main reason to always leave out OTP apps is stuff like sasl that has calls applications it doesn't depend on (the tools app).
• Option to exclude specific apps or warnings from the analysis. Example of the need for this is if the user wants to use the future "all dependencies" option and have a dep like elli that calls ssl but doesn't depend on it since the user doesn't have to use the ssl feature.
• Update the systools exref feature to make this being in relx no longer needed.

[tsloughter]

Since this seems to be checking by default

We aren't checking by default. It is default set to false.

[ferd]

Since this seems to be checking by default

We aren't checking by default. It is default set to false.

Yeah sorry, I meant "since this seems to be checking only undefines by default". I feel that's generally safe and good, and for a good while regretted not having it as a default in Dialyzer. I feel it's safer / less annoying to see for releases than dialyzer all of a sudden too.