[html2] template-dir option is not working

[lagebaue]

Description

The generator works fine without the template-dir option, but if it is set it crashes with the following error (doesn't even matter if there are any files in the folder or not, as long as the folder exists).

Exception in thread "Thread-1" java.lang.RuntimeException: Could not generate supporting file 'SupportingFile:
        templateFile: index.mustache
        folder:
        destinationFilename: index.html
'
        at io.swagger.codegen.v3.DefaultGenerator.generateSupportingFiles(DefaultGenerator.java:636)
        at io.swagger.codegen.v3.DefaultGenerator.generate(DefaultGenerator.java:763)
        at io.swagger.codegen.v3.cli.cmd.Generate.run(Generate.java:342)
        at java.lang.Thread.run(Unknown Source)
Caused by: java.io.FileNotFoundException: handlebars/htmlDocs2/index.mustache
        at com.github.jknack.handlebars.io.URLTemplateLoader.sourceAt(URLTemplateLoader.java:70)
        at com.github.jknack.handlebars.Handlebars.compile(Handlebars.java:357)
        at com.github.jknack.handlebars.Handlebars.compile(Handlebars.java:343)
        at io.swagger.codegen.v3.templates.HandlebarTemplateEngine.getHandlebars(HandlebarTemplateEngine.java:48)
        at io.swagger.codegen.v3.templates.HandlebarTemplateEngine.getRendered(HandlebarTemplateEngine.java:24)
        at io.swagger.codegen.v3.DefaultGenerator.generateSupportingFiles(DefaultGenerator.java:607)
        ... 3 more

Swagger-codegen version

3.0.8

Command line used for generation

$ java -jar ./dist/swagger-codegen-cli-3.0.8.jar generate -i docs/openapi.yaml -l html2 -o dist/html2 --template-dir docs/htmlDocs2

Steps to reproduce

1. execute the above command while template-dir is pointing to an existing directory

Related issues/PRs

This seems to be identical to this problem for the html generator.

[anselmos]

As mentioned in #9363 - try with adding this parameter: `--template-engine "mustache"
@lagebaue

[HugoMario]

@lagebaue , i merge this PR to solve this issue, i'm going to close it, but please let me know if if you still get the error to reopen it.