Fixed branch selection for excluding packages

[mnocon]

Follow-up to #55 , only 9eb669c constains new code (0b7f22c has already been reviewed)

The current branch selection logic is flawed:

• when PR is made to a branch called 1.3 then endpoint https://github.com/ibexa/$EDITION/1.3/composer.json is used - but https://github.com/ibexa/$EDITION/3.3/composer.json should be used instead.
• when PR is made to a branch called main then endpoint https://github.com/ibexa/$EDITION/main/composer.json is used - but https://github.com/ibexa/$EDITION/master/composer.json should be used instead.

It worked in ibexa/content#53 because the PR base branch (3.3) is the same as the endpoint that should be used - but it fails in https://github.com/ezsystems/ezplatform-version-comparison/pull/79

I thought I could avoid harcoding the branches by doing some magic with Github env variables, but it's not possible - hardcoding it seems like the easiest solution.

Tested in https://github.com/ezsystems/ezplatform-version-comparison/pull/79

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[micszo]

Will it require updating when new stable branch is created?

[mnocon]

Yes, I will do it when creating stable branches. We might be able to simplify it for v4 in the future:

• PR to main -> use master
• PR to branch other than main -> use the branch name
  because versioning is unified in v4

But right now I'm not 100% sure we can rely on this logic, so I'd like to merge this hardcoded way first to see how to works in practice