This change adds support for 'imports' - extra source files which contain
directives that can merged with 'main' source files. The purpose of course
is to support things like global usings or addTagHelpers, like
_ViewImports in MVC does today.
Instead of a one-off this is now a feature of the Razor langugage since
things like addTagHelper have an impact on the parsing behavior. Also,
having a standard imports concept keeps out feature creep, for instance
the 'global' usings we have today could really just be an import.
Imports allow single-line directives including the fundamental directives
like addTagHelper, using, and other friends. Code, content, and block
directives are not merged and will be ignored. We can consider making
these kinds of things warnings in the future.
4a49ee2164