Clean up dependencies

[renkun-ken]

Inspired by #469, it is likely that could eliminate those dependencies if we only rely on a tiny proportion of the functions provided in the package, or if they are easily replaceable.

For example, I could not find any usage of desc (search desc::) but we still import it?

[renkun-ken]

I go through the imported packages, looks like everything is fine except that desc is not used at all.

[renkun-ken]

Also, I'd like to move roxygen2 from Imports to Suggests like rmarkdown since

1. It has quite heavy dependencies.
2. We only use roxygen2:::parse_text().
3. It is optional and more useful for package developers who are very likely to have installed roxygen2.

[renkun-ken]

But it looks like most (if not all) dependencies of roxygen2 also appear in the dependencies of styler and lintr, then removing roxygen2 might not help reduce the installation size and time at all.

[randy3k]

But it looks like most (if not all) dependencies of roxygen2 also appear in the dependencies of styler and lintr, then removing roxygen2 might not help reduce the installation size and time at all.

Then I guess we could keep it.

[lorenzwalthert]

FWIW: Also note that {lintr} undergoes a dependency cut: r-lib/lintr#861