Show package description on hover for package dependencies

[VeryMilkyJoe]

It would be nice to be able to see a description of a dependency when hovering over that dependency in a cabal file.

Packages already have such a description field, the tricky part of this issue is to extract a given package's description from the correct package database. We should already have access to the package database, but we would need to be able to decide which package database corresponds to the current cabal file.

[pbrinkmeier]

I am currently working on this and #4416. As a first approximation we are going to call out to cabal info <package> when hovering a dependency. In the future we might be able to coax Cabal to provide this information through its API.

As a reference for the cabal info output format, I'll be using the implementation in cabal-install.

[pbrinkmeier]

My current impl simply dumps the cabal info output into the tooltip

Example output for cabal info text:

* text             (library)
    Synopsis:      An efficient packed Unicode text type.
    Versions available: 0.11.3.1, 1.1.1.4, 1.2.4.1, 1.2.5.0, 2.0, 2.0.1, 2.0.2,
                        2.1, 2.1.1 (and 70 others)
    Versions installed: 2.0.2
    Homepage:      https://github.com/haskell/text
    Bug reports:   https://github.com/haskell/text/issues
    Description:   An efficient packed, immutable Unicode text type (both strict
                   and lazy).

                   The 'Text' type represents Unicode character strings, in a
                   time and space-efficient manner. This package provides text
                   processing capabilities that are optimized for performance
                   critical use, both in terms of large data quantities and high
                   speed.

                   The 'Text' type provides character-encoding, type-safe case
                   conversion via whole-string case conversion functions (see
                   "Data.Text"). It also provides a range of functions for
                   converting 'Text' values to and from 'ByteStrings', using
                   several standard encodings (see "Data.Text.Encoding").

                   Efficient locale-sensitive support for text IO is also
                   supported (see "Data.Text.IO").

                   These modules are intended to be imported qualified, to avoid
                   name clashes with Prelude functions, e.g.

                   > import qualified Data.Text as T

                   == ICU Support

                   To use an extended and very rich family of functions for
                   working with Unicode text (including normalization, regular
                   expressions, non-standard encodings, text breaking, and
                   locales), see the [text-icu
                   package](https://hackage.haskell.org/package/text-icu) based
                   on the well-respected and liberally licensed [ICU
                   library](http://site.icu-project.org/).
    Category:      Data, Text
    License:       BSD-2-Clause
    Author:        Bryan O'Sullivan 
    Maintainer:    Haskell Text Team , Core Libraries Committee
    Source repo:   https://github.com/haskell/text
    Flags:         developer, simdutf, pure-haskell
    Dependencies:  array >=0.3 && <0.6, base >=4.10 && <5, binary >=0.5 && <0.9,
                   bytestring >=0.10.4 && <0.13, deepseq >=1.1 && <1.6,
                   ghc-prim >=0.2 && <0.12, template-haskell >=2.5 && <2.23,
                   system-cxx-std-lib ==1.0, base <0,
                   data-array-byte >=0.1 && <0.2, QuickCheck >=2.12.6 && <2.15,
                   base <5, bytestring, deepseq, directory, ghc-prim, tasty,
                   tasty-hunit, tasty-quickcheck, template-haskell,
                   transformers, text, data-array-byte >=0.1 && <0.2,
                   tasty-inspection-testing, base, bytestring >=0.10.4,
                   containers, deepseq, directory, filepath, tasty-bench >=0.2,
                   text, transformers
    Documentation: [ Not installed ]
    Cached:        No
    Modules:
        Data.Text
        Data.Text.Array
        Data.Text.Encoding
        Data.Text.Encoding.Error
        Data.Text.Foreign
        Data.Text.IO
        Data.Text.Internal
        Data.Text.Internal.Builder
        Data.Text.Internal.Builder.Functions
        Data.Text.Internal.Builder.Int.Digits
        Data.Text.Internal.Builder.RealFloat.Functions
        Data.Text.Internal.ByteStringCompat
        Data.Text.Internal.Encoding
        Data.Text.Internal.Encoding.Fusion
        Data.Text.Internal.Encoding.Fusion.Common
        Data.Text.Internal.Encoding.Utf16
        Data.Text.Internal.Encoding.Utf32
        Data.Text.Internal.Encoding.Utf8
        Data.Text.Internal.Fusion
        Data.Text.Internal.Fusion.CaseMapping
        Data.Text.Internal.Fusion.Common
        Data.Text.Internal.Fusion.Size
        Data.Text.Internal.Fusion.Types
        Data.Text.Internal.IO
        Data.Text.Internal.Lazy
        Data.Text.Internal.Lazy.Encoding.Fusion
        Data.Text.Internal.Lazy.Fusion
        Data.Text.Internal.Lazy.Search
        Data.Text.Internal.PrimCompat
        Data.Text.Internal.Private
        Data.Text.Internal.Read
        Data.Text.Internal.Search
        Data.Text.Internal.StrictBuilder
        Data.Text.Internal.Unsafe
        Data.Text.Internal.Unsafe.Char
        Data.Text.Lazy
        Data.Text.Lazy.Builder
        Data.Text.Lazy.Builder.Int
        Data.Text.Lazy.Builder.RealFloat
        Data.Text.Lazy.Encoding
        Data.Text.Lazy.IO
        Data.Text.Lazy.Internal
        Data.Text.Lazy.Read
        Data.Text.Read
        Data.Text.Unsafe