Improvement: Single source of version truth

Description

Note: This ticket is purely about implementation. Currently the version numbers are determined via multiple ways depending on the config and the called method. Depending if get_current_version, get_next_version, and more, different ways to determine this version are used: based on tags, based on parsed commit messages, the version file (and maybe more?). I understand the need for this in the current implementation, but it feels a bit error prone and I think it would make the code easier and cleaner if we could come up with a single way how versions are determined / parsed / found.

I would like to open a discussion about this…

[jacksbox]

@danth jup, tags would need to always be created. I think that is a fair requirement (in the future we could think of a plugin solution to enable different and more complex version sources)…

This breaks with a feature I introduced as well -> https://github.com/relekang/python-semantic-release/pull/341, but as stated in the PR, this was for the usecase of monorepos, something this package is currently not made for and also probably should not be used for. So I would be fine with removing this features again. (the whole prerelease feature seems to be a lot more useful and for a bigger audience as well)

[bernardcooke53]

This is definitely happening in #487 - it will be tags only. @nejch the Gitlab implementation in PSR is definitely lagging behind Github and Gitea - would be great to have your input!