We shouldn't be attempting to append SourceFiles
to the module after-the-fact for syntactic macro
expansion, refactor things such that the SourceFile
is created alongside the ModuleDecl.
Expand macros in the specified source file syntactically (without any
module imports, nor typechecking).
Request would look like:
```
{
key.compilerargs: [...]
key.sourcefile: <file name>
key.sourcetext: <source text> (optional)
key.expansions: [<expansion specifier>...]
}
```
`key.compilerargs` are used for getting plugins search paths. If
`key.sourcetext` is not specified, it's loaded from the file system.
Each `<expansion sepecifier>` is
```
{
key.offset: <offset>
key.modulename: <plugin module name>
key.typename: <macro typename>
key.macro_roles: [<macro role UID>...]
}
```
Clients have to provide the module and type names because that's
semantic.
Response is a `CategorizedEdits` just like (semantic) "ExpandMacro"
refactoring. But without `key.buffer_name`. Nested expnasions are not
supported at this point.
