[FLORA-496] Show the system libraries required by the cabal file – part 1

[tchoutri]

Proposed changes

This PR implements the first part of showing system libraries required by packages, as declared in cabal files. Right now, it is limited to extra-libraries stanza.

Due to limitations of the cabal parser, common stanzas are not shown. This means that extra-libraries stanzas in common stanzas are invisible to Flora, for the time being.

Contributor checklist

• [x] My PR is related to #496
• [ ] I have read and understood the CONTRIBUTING guide
• [ ] I have inserted my change and a link to this PR in the CHANGELOG