Add using directive for specifying a (default) main class

[fourth44]

resolves #370.
All the infrastructure for passing a main class in the build options already existed, so this only adds the directive to fill that option.

[lwronski]

@fourth44 Thank you for your contribution! I have just left one suggestion.

[lwronski]

I think, we should not allowed to passing main class few times.

You can use SeveralMainClassesFoundError which is already defined here.

[fourth44]

I'm happy to implement your suggestion, but I would like to point out that SeveralMainClassesFoundError seems to be intended for the number of main classes found by analyzing the classpath, while this situation is about the number of main classes supplied by directives. That said, supplying multiple --main-class command line arguments also results in an error so this should definitely too, and it seems best to reuse this error rather than be pedantic that it's not about found but supplied main classes.

[lwronski]

I think too, that is better to reuse SeveralNoClassdefFoundError in this case.

Anyway, we started working on an options validation mechanism, which should detect when a directive is specified multiple times, so for now your proposal I good to merge. Later, we will integrate a new mechanism with your directive.

[SethTisue]

is there a ticket about kebab-case vs camelCase for directives? it may seem bike-shed-y, but it's the sort of decision that's hard to change later, too

[lwronski]

is there a ticket about kebab-case vs camelCase for directives? it may seem bike-shed-y, but it's the sort of decision that's hard to change later, too

Hi @SethTisue, Here is opened the discussion to unify standard for naming convention in directives.