add support for start_dir easyconfig template

[Flamefire]

As #4035 went stale with some issues remaining I tackled this. The test was quite tricky as this template can only be set once the source is extracted. Hence I wrote the test so that it should be set in the configure and build step of a "regular" EasyConfig and in an extension which is where this will most likely be used.

The extension is very tricky:

• We must not resolve the start_dir in the options during __init__ -> Move the relevant code down to the Extension class and clear the (new) template too
• The template value is then later set in a method I introduced in #3426

@jfgrimm Please have a look here.