[ts-interface-generator] Support TS ${configDir} in path mappings for source exports

[stabpd]

Describe the bug
Source exports are not or incorrectly resolved when using TS config file template variable ${configDir} in path mappings.

// tsconfig.json
{
    "compilerOptions": {
        // [...]
        "paths": {
            "mylib/*": ["${configDir}/src/mylib/*"] // <- PROBLEM: This is resolved to an absolute path by TS.
                                                    // Joining basePath with pathMattingTarget does not work in that case.
                                                    // ("./src/mylib/*" works though)
        }
    },
    "include": ["${configDir}/src/**/*"] // <- no issue here
}

That then results in an error when trying to resolve imports for interfaces.

For the type 'mylib.MyEnum' an import is created with module name 'mylib/MyEnum', using its default export. Is this correct?
Usually this indicates some kind of issue. Maybe this import will also show up as error in your code editor.

Expected behavior
I'd expect the same behaviour for the same path mapping targets, no matter if specified relatively or with ${configDir}.

Additional context
ts-interface-generator expects relative path mapping targets and applies path.join in packages/ts-interface-generator/src/addSourceExports.ts#L39.

Using path.resolve instead seems to fix the issue for me although I do not know

1. if this introduces other issues and
2. if other changes would be needed to fully support ${configDir} in all places / features.

- const fullTargetPath = path.join(basePath, allPathMappings[i].target);
+ const fullTargetPath = path.resolve(basePath, allPathMappings[i].target);

Debugged values with current implementation and ${configDir}:

variable	value
basePath	/my/project/root
allPathMappings[i].target	/my/project/root/src/mylib/
fullTargetPath	/my/project/root/my/project/root/src/mylib/ ⚠️
globalExportName	undefined.MyEnum ⚠️