Preserve optional dependencies in output of `pigar check`

[Gulshan Singh]

Is your feature request related to a problem? Please describe.
I use pigar check --format-requirements to update my requirements.txt. This works fine for most projects, but sometimes I have a package with optional dependencies, and pigar doesn't include this in the output. For example, if the requirements.txt is:

coverage[toml]==7.4.2

Running pigar check --format requirements correct updates the package version, but it drops the optional toml dependency.

coverage==7.4.3

Describe the solution you'd like
Preserve any optional dependencies. In the example above, the output should be:

coverage[toml]==7.4.3