Support elseif/else directive

[fummicc1]

Note

This PR is made after this closed PR that has some faults for parsing complicated if-compiler-directive.

Motivation

Mockolo didn't support elseif compiler directive as of v2.3.1.

There are some bugs that mockolo faces with:

• Both of poundIf and poundElseIf are treated as poundIf
• poundElse is ignored.

Approach

This PR's main updates consist of two parts:

1. Make ImportMap more structured by introducing ParsedImports type.
2. Add IfMacroModel to Clause type and store entities to Clause type rather than IfMacroModel.

Detail

ImportHandler

mockolo provides a feature that user can inject any import statement via mockolo CLI what we call customImports.

This is one of the reason that we can't collect all imports just leveraging swift-syntax.

Operation to merge all-imports is performed in handleImports function, and the function removes duplicated imports and recursively renders conditional import statements.

IfMacroModel

This PR added Clause type to IfMacroModel that represents if compiler directive in @mockable entity, and now entities are stored by Clause type.
The order of if-elseif-else is decided by index of IfConfigDeclSyntax.clauses.enumerated(), and compilerDirectiveKey in previous PR was not actually needed.

[fummicc1]

[self review]

Except for Import, all nodes within IfDeclConfigSyntax are stored in IfMacroModel's entities.
However, import statement is not represented as one of them, rather it is represented as ImportContent.

[fummicc1]

[self review]

Now entities property is moved from IfMacroModel to IfMacroModel.Clause so that we can know which compiler directive stores them.

Because IfMacroModel self is also Model, this recursively stores.

[fummicc1]

[self review]

Because other types such as ImportMap are public scope, Import should be public as well.