perf(gatsby): don't create Head modules in develop if it's not used

[pieh]

Description

Right now, in develop we create Head modules for every page template even if that template doesn't actually implement Head. Theorhetically this would be fine, but on larger projects this result in quite an increase of needed memory by webpack compilation, so this PR is crude way to limit impact of it by allowing memory usage be more in line with what it used to be before Head API was introduced.

As we do want people to use that API over alternatives we will have to figure out how memory usage can be optimized when Head is actually implemented by templates

Documentation

Related Issues

#36899

[pieh]

The failing tests uncovered at least some issues in file-parser which doesn't find all the exports (depending on syntax), so will be looking into it more now