exec-env improvements

[scop]

See individual commit messages for details on each change.

[missingcharacter]

Checking if GOROOT and GOPATH should be more than enough, there is no need to add unset or add 2 environment variables (ASDF_GOLANG_DISABLE_GOROOT, ASDF_GOLANG_DISABLE_GOPATH

[missingcharacter]

I just finished reading https://github.com/asdf-community/asdf-direnv/issues/149, I think GOPATH should not be changed if it was already set since GOPATH is normally a developer's workspace.

but you are right about unsetting/setting GOROOT depending on the switch between versions

[scop]

I agree changing is more crucial for GOROOT if it's set.

For GOPATH, one approach would be to change if the already set value looks like something asdf-golang has set it to earlier, otherwise leave alone.

Anyway, I can revise this PR once I know what behavior the maintainers here prefer.

Personally, I would drop touching both GOROOT and GOPATH altogether, no matter if set or not. That'd be a backwards incompatible change though -- therefore the ASDF_GOLANG_DISABLE_* variables, to make the breaking change opt-in.

[kennyp]

Thanks for the PR @scop! I've got the next two weeks off, so I'll spend some time digging into this one.

[scop]

Rebased to resolve conflicts, cleaned up indentation some.

[scop]

Anything I can do to help with this?