Deduplicate packages in the install plan after the solver removes qualifiers.

Parsec parser

Initialize all PackageDescription fields in solver DSL.

Note: we notify only about nbsp at the beginning of the line
nbsp inside fields get just parsed atm

- Also print name of index file

- encode . lenientDecode prepass
- test on 01-index.tar.gz
- few additional files are failing for weird reason

- Initial import of stuff
- Add hackage index parsing test for parsec
- We can parse name and version
- Add generics-sop dep
- Use D.C.Prelude
- Add StructDiff
- Fix integral
- Add Parsec -class
- Add licence parsing
- Free text fields (parsed as ascii atm)
- Better StructDiff
- Parsec BuildType
- Implement VersionRange parser
- Section parsing first steps
- Sketch parseCondTree
- more sec parsing
- Module name
- hs-source-dirs
    ::: composition/0.1/composition.cabal
    ::: composition/0.2/composition.cabal
    ::: composition/0.2.1/composition.cabal
    ::: composition/1.0.0.0/composition.cabal
- PError (Position 20 1) "Unknown section: \"source-repository\""
- Parses condition
- Add some notes, address few comments
- Install alex on linux travis
- Make ParseResult strict state monad
- Use withinVersion
- No warnings
- Move to Distribution.PackageDescription.Parsec.FieldDescr
- extensions
- SourceRepo (sans kind) parsing
- RepoKind
- Few more stuff
- Add Distribution.Compat.Parsec
- We can parse all 'composition*' packages cabal files \o/
- fromUtf8BS
- Clean up abit
- More stuff
- And more stuff
- Traling whitespace :(
- Executables
- Benchmarks
- spaceListField
- Parse flag sections
- a ~ Char,
- tested with
- package description x-fields
- Conditions
- reformat
- Handle old syntax
- More fields
- More stuff
- ^co ok,
- more stuff
- ^c ok
- some of hackage ok
- A-Z ok
- Works [A-al)
- to the h
- rest of Hackage
- Introduce parsec -flag
- Count warnings
- Verify we get not less warnings from parsec parser
- fixup! Introduce parsec -flag
- Warn about old/new syntax and cabal-version
- Invalid UTF warning
- Deprecated fields
- Fix meta
- Move transformers dep under parsec flag
- Add parsec travis job

Better error messages

This is more consistent with the way that the parser creates
GenericPackageDescriptions.

Fixes #4026

Drop version check when resolving package names.

Force gen-extra-source-files to use pre-installed library.

In #4017, hvr reported that when he used --allow-older/--allow-newer,
there was an assert failure in toConfiguredComponent.  Indeed
the problem was that toConfiguredComponent was testing version
ranges of build-depends to determine which package to select, but
there was no satisfying one (since the build-depends field had
not been updated.)

After thinking about this for a bit, it seemed a bit bogus for
us to be doing another version check at this late phase; we
already picked dependencies earlier in the configuration process.
So I decided to drop it.

To drop it, however, I needed to remove support for a feature (discussed
in #4020), which uses version ranges to disambiguate whether or not a
dependency is on an external package or an internal package.  This
feature doesn't seem to be very useful.  If someone asks, I'll
check on Hackage to see if anyone is using it.

Also added some useful extra debug info.

Fixes #4020 and #4017

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>