Improvements to support type-hinting

When attempting to use the output functions in plugins, mypy was warning
about calling untyped functions. The changes to output.py should resolve
this.

It's also necessary to import the ImportGraph protocol when creating a
plugin, so it makes sense that this should be exported at the top level
too. This avoids needing to reach deep into the package.

src/importlinter/__init__.py

@@ -3,3 +3,7 @@
 from .application import output  # noqa
 from .domain import fields  # noqa
 from .domain.contract import Contract, ContractCheck  # noqa
+from importlinter.domain.ports.graph import ImportGraph  # noqa
+
+
+__all__ = ["output", "fields", "Contract", "ContractCheck", "ImportGraph"]

src/importlinter/application/output.py

@@ -46,13 +46,13 @@ def print(
         """
         self.printer.print(text, bold, color, newline)
 
-    def indent_cursor(self):
+    def indent_cursor(self) -> None:
         """
         Indents the cursor ready to print a line.
         """
         self.printer.print(" " * INDENT_SIZE, newline=False)
 
-    def new_line(self):
+    def new_line(self) -> None:
         """
         Print a blank line.
         """
@@ -84,19 +84,19 @@ def print_heading(self, text: str, level: int, style: Optional[str] = None) -> N
         self.printer.print(heading_line, bold=is_bold, color=color)
         self.printer.print()
 
-    def print_success(self, text, bold=True):
+    def print_success(self, text: str, bold: bool = True) -> None:
         """
         Prints a line to the console, formatted as a success.
         """
         self.printer.print(text, color=COLORS[SUCCESS], bold=bold)
 
-    def print_error(self, text, bold=True):
+    def print_error(self, text: str, bold: bool = True) -> None:
         """
         Prints a line to the console, formatted as an error.
         """
         self.printer.print(text, color=COLORS[ERROR], bold=bold)
 
-    def print_warning(self, text):
+    def print_warning(self, text: str) -> None:
         """
         Prints a line to the console, formatted as a warning.
         """