Don't create spurious dirs on init

[emilypi]

This gets rid of that pesky exe dir being created when we specify libraries interactively:

λ P temp → λ git ignore-app-dir-for-libraries → ../../dist-newstyle/build/x86_64-osx/ghc-8.10.3/cabal-install-3.5.0.0/x/cabal/noopt/build/cabal/cabal init 
...
What does the package build:
   1) Executable
   2) Library
   3) Library and Executable
Your choice? 2
...
Generating LICENSE...
Generating CHANGELOG.md...
Generating src/MyLib.hs...
Generating test/MyLibTest.hs...
Generating temp.cabal...

Warning: no synopsis given. You should edit the .cabal file and add one.
You may want to edit the .cabal file and add a Description field.
λ P temp → λ git ignore-app-dir-for-libraries → ls
CHANGELOG.md LICENSE      src          temp.cabal   test

and for Libraries + Executables,

λ P temp → λ git ignore-app-dir-for-libraries → ../../dist-newstyle/build/x86_64-osx/ghc-8.10.3/cabal-install-3.5.0.0/x/cabal/noopt/build/cabal/cabal init 
Should I generate a simple project with sensible defaults? [default: y] n
What does the package build:
   1) Executable
   2) Library
   3) Library and Executable
Your choice? 3
...
Generating LICENSE...
Generating CHANGELOG.md...
Generating src/MyLib.hs...
Generating exe/Main.hs...
Generating test/MyLibTest.hs...
Generating temp.cabal...

Warning: no synopsis given. You should edit the .cabal file and add one.
You may want to edit the .cabal file and add a Description field.
λ P temp → λ git ignore-app-dir-for-libraries → ls
CHANGELOG.md LICENSE      exe          src          temp.cabal   test

and for Executables,

λ P temp → λ git ignore-app-dir-for-libraries* → ../../dist-newstyle/build/x86_64-osx/ghc-8.10.3/cabal-install-3.5.0.0/x/cabal/noopt/build/cabal/cabal init
Should I generate a simple project with sensible defaults? [default: y] n
What does the package build:
   1) Executable
   2) Library
   3) Library and Executable
Your choice? 1
...
Generating LICENSE...
Generating CHANGELOG.md...
Generating exe/Main.hs...
Generating temp.cabal...

Warning: no synopsis given. You should edit the .cabal file and add one.
You may want to edit the .cabal file and add a Description field.
λ P temp → λ git ignore-app-dir-for-libraries* → ls
CHANGELOG.md LICENSE      exe          temp.cabal

---

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• [?] Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.

[emilypi]

Question about the changelog: master seems to only be as recent as 3.2. Do you want to just cherrypick this, or should i add an entry?

[fgaz]

master seemes to only be as recent as 3.2

@emilypi what do you mean?

[emilypi]

@fgaz the changelog for cabal-install ends with 3.2. It doesn't seem to have been maintained. https://github.com/haskell/cabal/blob/master/cabal-install/changelog

[fgaz]

I think that's because 3.2 is still the last release and 3.4 is still at the rc stage. @phadej?

[emilypi]

sourceDirs = Nothing in baseFlags was actually the wrong state for this to be in. Throughout the course of init it actually gets turned into Just [] for Executables. So, to test against this, we set this as our golden data.

[fgaz]

Could you split the TESTING.md stuff into a separate commit?

[emilypi]

@fgaz split up for you.

[emilypi]

I found a few more bugs while going through the code and wrote up a proposal fix for #7265. In case that doesn't get done by the time 3.4 is released, I think this would still be a good quality of life improvement for this particular function.

[fgaz]

Thanks for splitting the commit!
Please replace the current subject lines of the commit messages (commit x of n) with short summaries of the changes

[emilypi]

@fgaz can I get a 👍 ?

[fgaz]

👍 👍

Remember to add the changelog entry before merging!

[emilypi]

Thanks @fgaz! Added, and merging.

[ulysses4ever]

Does this merge mean that #6772 is fixed (in master) now? Should it be closed?

[emilypi]

Yes, good call @ulysses4ever