Fix --project-file behaviour.

- This commit sets the ProjectRoot consistently for both absolute
  and relative --project-file pats. The ProjectRoot is set as the
  directory where the --project-file is located.
- Before this change relative project files are rooted to where the
  project file is resolved, when recursively searching upward from
  the current directory, and not where it is located.

  Previous behaviour is as follows,

  gitpod ~/0/1/2/foo $ tree ~/0
  /home/gitpod/0
  └── 1
      ├── 2
      │   └── foo
      │       ├── app
      │       │   └── Main.hs
      │       ├── CHANGELOG.md
      │       └── foo.cabal
      └── cabal.project

  and inside foo let's run

  $ pwd
  /home/gitpod/0/1/2/foo
  $ cabal build all --project-file ../cabal.project

  which builds the project setting the project root at 2 using
  the project file at 1

  $ ls ~/0/1 ~/0/1/2
  /home/gitpod/0/1:
  2  cabal.project

  /home/gitpod/0/1/2:
  dist-newstyle foo

  However, if the cabal.project file was instead in 0 directory,
  cabal would fail to build since the resolved project root is at
  1 and cabal fails to find a package at 1/foo

  gitpod ~/0/1/2/foo $ mv ~/0/1/cabal.project ~/0/
  gitpod ~/0/1/2/foo $ cabal build all --project-file ../cabal.project
  When using configuration(s) from /home/gitpod/0/1/../cabal.project, the following errors occurred:
  The package location 'foo' does not exist.

  This beaviour is almost never what we want.

cabal-install/src/Distribution/Client/ProjectConfig.hs

@@ -406,8 +406,7 @@ resolveBuildTimeSettings verbosity
 findProjectRoot :: Maybe FilePath -- ^ starting directory, or current directory
                 -> Maybe FilePath -- ^ @cabal.project@ file name override
                 -> IO (Either BadProjectRoot ProjectRoot)
-findProjectRoot _ (Just projectFile)
-  | isAbsolute projectFile = do
+findProjectRoot _ (Just projectFile) = do
     exists <- doesFileExist projectFile
     if exists
       then do projectFile' <- canonicalizePath projectFile
@@ -416,13 +415,13 @@ findProjectRoot _ (Just projectFile)
               return (Right projectRoot)
       else return (Left (BadProjectRootExplicitFile projectFile))
 
-findProjectRoot mstartdir mprojectFile = do
+findProjectRoot mstartdir Nothing = do
     startdir <- maybe getCurrentDirectory canonicalizePath mstartdir
     homedir  <- getHomeDirectory
     probe startdir homedir
   where
     projectFileName :: String
-    projectFileName = fromMaybe "cabal.project" mprojectFile
+    projectFileName = "cabal.project"
 
     -- Search upwards. If we get to the users home dir or the filesystem root,
     -- then use the current dir
@@ -431,9 +430,7 @@ findProjectRoot mstartdir mprojectFile = do
       where
         go :: FilePath -> IO (Either BadProjectRoot ProjectRoot)
         go dir | isDrive dir || dir == homedir =
-          case mprojectFile of
-            Nothing   -> return (Right (ProjectRootImplicit startdir))
-            Just file -> return (Left (BadProjectRootExplicitFile file))
+          return (Right (ProjectRootImplicit startdir))
         go dir = do
           exists <- doesFileExist (dir </> projectFileName)
           if exists

doc/cabal-project.rst

@@ -258,10 +258,14 @@ package, and thus apply globally:
     ``--project-file=my.project``, then the other files that will
     be probed are ``my.project.freeze`` and ``my.project.local``.
 
-    If the specified project file is a relative path, we will
-    look for the file relative to the current working directory,
-    and then for the parent directory, until the project file is
-    found or we have hit the top of the user's home directory.
+    The location of the ``--project-file`` also dictates the project
+    root with respect to which all other locations, like packages, are
+    resolved.
+
+    If a project file is not specified, we will look for the file relative
+    to the current working directory, and then for the parent directory,
+    until the project file is found or we have hit the top of the user's
+    home directory.
 
     This option cannot be specified via a ``cabal.project`` file.