Use runtime/debug for providing the version

[sapk]

Since go1.12 we can get the version from runtime/debug for the main module (gitea itself) and also submodules (lib) we are using. golang/go@45e9c55

So when we will only support 1.12 and 1.13 we could migrate to this system and clear some makefile and also wiil fix #4782 since we will not rely on env variable.

[zeripath]

This is probably fixed by #6836 and its backport #6839

[sapk]

This isn't the same. I suggest to use the fact that the module version (tag) is now available when building with mod. This will remove any need for a lot of line in makefile.

[wxiaoguang]

I do not think Go's module version works well nor it satisfies Gitea's version mechanism.

https://github.com/golang/go/issues/29228

Correct me if I am wrong, feel free to re-open if there are more details or feasible plans.