fix: module filters are not discovered when using route filters

[kenjis]

Description
Fixes #4720

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• [] User guide updated
• Conforms to style guide

[iRedds]

Yes, this solution works.
But what if we move the search for filters to the class constructor?

[kenjis]

@iRedds I moved it. Does it work?

[iRedds]

@kenjis You asks me? How about tests? =)

[kenjis]

@iRedds Yes, of course. You are the only one user for this functionality in this planet.
Most important. :-)

[kenjis]

@michalsn #3459 (comment)

Is there something wrong with moving$this->discoverFilters() back to the constructor?

I think the Filters files in the namespace are determined at the time of the instance creation,
so it seems better to execute it once in the constructor.

[michalsn]

I think this is a good fix.

Can we add a test for this to make sure it won't be moved around anymore?

[kenjis]

@michalsn Thanks! I added a test.

[michalsn]

Looks good to me.