Custom templates don't work on Windows

Description

Custom templates for supporting files don't get resolved under Windows. The reason is that DefaultGenerator.generateSupportingFiles (around line 581 in version 3.0.0-rc1) contains the following erroneous construct:

	if(ignoreProcessor.allowsFile(new File(outputFilename))) {
		if (templateFile.endsWith("mustache")) {
			String templateName = templateFile;
			final com.github.jknack.handlebars.Template hTemplate = getHandlebars(templateName.replace(config.templateDir(), StringUtils.EMPTY)); // 581: WRONG!!!

On Windows, config.templateDir() returns the canonical Windows path of the custom template directory (e.g. C:\source\svn\WBPM\cloud\obj-api\trunk\objectapi\templates\jaxrs-cxf) whereas templateName has already been converted (at around line 568, which changes \ to / in templateFile) to use Unix-style path separator and has a value like C:/source/svn/WBPM/cloud/obj-api/trunk/objectapi/templates/jaxrs-cxf/server/pom.mustache. Thus the call to getHandlebars() fails with the following exception:

java.io.FileNotFoundException: C:\source\svn\WBPM\cloud\obj-api\trunk\objectapi\templates\jaxrs-cxf/C:/source/svn/WBPM/cloud/obj-api/trunk/objectapi/templates/jaxrs-cxf/server/pom.mustache

I discovered this while trying to use swagger-codegen-maven-plugin to generate jaxrs-cxf code with custom templates (BTW the existing jaxrs-cxf templates do not yield an executable Spring application).

Isn't it rather a major bug that custom templates don't work on Windows?!

Swagger-codegen version

3.0.0-rc1

Swagger declaration file content or url

Sorry, I am not permitted to post the YAML in question, for reasons of commercial confidentiality. Pretty much any old YAML should suffice.

Command line used for generation

swagger-codegen-maven-plugin configuration:

	<configuration>
		<language>jaxrs-cxf</language>
		<templateDirectory>${project.basedir}/templates/jaxrs-cxf</templateDirectory>
		<inputSpec>${project.build.yaml.directory}/ae.yaml</inputSpec>
		<output>${project.build.output.directory}/jaxrs-cxf</output>
		<groupId>com.blah.bpm.clients</groupId>
		<artifactId>objectapi-test-server</artifactId>
		<artifactVersion>${objectapi.version}</artifactVersion>
		<apiPackage>com.blah.clients.ae.api</apiPackage>
		<modelPackage>com.blah.clients.ae.model</modelPackage>
		<invokerPackage>ae</invokerPackage>
		<configOptions>
			<sourceFolder>src/main/java</sourceFolder>
			<generateSpringApplication>true</generateSpringApplication>
			<generateSpringBootApplication>true</generateSpringBootApplication>
			<artifactName>Blah Microservices</artifactName>
		</configOptions>
	</configuration>

Steps to reproduce

Run mvn install on a project configured to invoke swagger-codegen-maven-plugin configured as above.

Related issues/PRs

I had to patch in a fix for the following bug to get this far:
https://github.com/swagger-api/swagger-codegen/issues/8291

Suggest a fix/enhancement

Please see the description. I don't have a suggested fix at this point.

Start in DefaultGenerator.generateSupportingFiles around lines 568-581 and inspect how templateFile and config.templateDir() are combined before getHandlebars(). Reproduce the Maven-plugin generation with a custom template directory on Windows, then verify that custom supporting-file templates resolve without the duplicated path shown in the exception.