Resolves: multiple packages build

When there are multiple packages with the "from" keyword, the poetry build command, doesn't install the packages as expected.
Ex:

packages = [
    { include = "XXX", from = "path/to/dir1" },
    { include = "YYY", from = "path/to/dir2" },
    { include = "ZZZ", from = "path/to/dir3" },
    { include = "A" }
]

In fact, when poetry build, its find all the files and directories inside the path specified by the keyword 'from' ('find_files_to_add' function from poetry/core/masonry/builders/builder.py#L157) .
Next, if it's a directory, it find all files and add them with the 'BuildIncludeFile' class using the current path (self._path).
BuildIncludeFile with self._path
poetry/core/masonry/builders/builder.py#L176-L180

 include_file = BuildIncludeFile(
     path=current_file,
     project_root=self._path,
     source_root=self._path,
 )

If it's a file, it add it with the 'BuildIncludeFile' BUT using the 'specified from keyword' (include.base) if it is defined.
BuildIncludeFile with include.base
poetry/core/masonry/builders/builder.py#L188-L199

  if (
      isinstance(include, PackageInclude)
      and include.source
      and self.format == "wheel"
  ):
      source_root = include.base
  else:
      source_root = self._path


  include_file = BuildIncludeFile(
      path=file, project_root=self._path, source_root=source_root
  )

This Pr is intented to use the 'include.base' when it's the directory (if from is defined).
So only move up to the line #L172

  if (
      isinstance(include, PackageInclude)
      and include.source
      and self.format == "wheel"
  ):
      source_root = include.base
  else:
      source_root = self._path

And use source_root=source_root for the 'BuildIncludeFile' class in the directory part (#L179).

• Added tests for changed code.
• Updated documentation for changed code.