cleanup: fix typings for dependencies.py and external_commands.py

[shubhbapna]

part of #226

Fixes the following errors:

src/fromager/dependencies.py:33: error: Incompatible return value type (got "set[Requirement] | None", expected "set[Requirement]")  [return-value]
src/fromager/dependencies.py:73: error: Returning Any from function declared to return "Iterable[Requirement]"  [no-any-return]
src/fromager/dependencies.py:90: error: Incompatible return value type (got "set[Requirement] | None", expected "set[Requirement]")  [return-value]
src/fromager/dependencies.py:119: error: Returning Any from function declared to return "set[Requirement]"  [no-any-return]
src/fromager/dependencies.py:136: error: Incompatible return value type (got "set[Requirement] | None", expected "set[Requirement]")  [return-value]
src/fromager/dependencies.py:165: error: Returning Any from function declared to return "set[Requirement]"  [no-any-return]
src/fromager/dependencies.py:172: error: Argument 1 to "Wheel" has incompatible type "Path"; expected "str"  [arg-type]
src/fromager/dependencies.py:173: error: Argument 2 to "_filter_requirements" has incompatible type "Sequence[str]"; expected "Iterable[Requirement]"  [arg-type]
src/fromager/dependencies.py:207: error: Missing type parameters for generic type "dict"  [type-arg]
src/fromager/dependencies.py:222: error: Missing type parameters for generic type "dict"  [type-arg]
src/fromager/dependencies.py:239: error: Missing type parameters for generic type "dict"  [type-arg]
src/fromager/dependencies.py:243: error: Function is missing a type annotation  [no-untyped-def]
src/fromager/dependencies.py:262: error: Function is missing a return type annotation  [no-untyped-def]

src/fromager/external_commands.py:13: error: Missing type parameters for generic type "dict"  [type-arg]

fixes in dependencies.py needed fixes in external_commands.py.

Unfortunately we cannot upgrade pyproject_hooks to 1.1.0 to use their types since #226 (comment)
So the follow errors cannot be fixed:

src/fromager/dependencies.py:119: error: Returning Any from function declared to return "Iterable[str]"  [no-any-return]
src/fromager/dependencies.py:165: error: Returning Any from function declared to return "Iterable[str]"  [no-any-return]