Fix editable package test failing on windows.

[jvansch1]

Summary: In the previous implementation this editable package feature, we hard-coded a path prefix which did not work in windows environments. Now we programatically parse this URL instead of relying on a hard-coded string.

Differential Revision: D91489776

[meta-codesync]

@jvansch1 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D91489776.

[rchen152]

Review automatically exported from Phabricator review in Meta.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

openlibrary (https://github.com/internetarchive/openlibrary)
+ ERROR openlibrary/plugins/openlibrary/lists.py:811:43-813:18: Cannot set item in `dict[str, list[dict[Unknown, Unknown]]]` [unsupported-operation]
+ ERROR openlibrary/plugins/openlibrary/lists.py:818:40-88: Cannot set item in `dict[str, list[dict[Unknown, Unknown]]]` [unsupported-operation]
+ ERROR openlibrary/plugins/openlibrary/lists.py:823:42-825:18: Cannot set item in `dict[str, list[dict[Unknown, Unknown]]]` [unsupported-operation]
+ ERROR openlibrary/plugins/upstream/tests/test_merge_authors.py:142:12-52: Object of class `NoneType` has no attribute `key` [missing-attribute]
+ ::error file=openlibrary/plugins/openlibrary/lists.py,line=811,col=43,endLine=813,endColumn=18,title=Pyrefly unsupported-operation::Cannot set item in `dict[str, list[dict[Unknown, Unknown]]]`%0A  Argument `list[Thing | Unknown]` is not assignable to parameter `value` with type `list[dict[Unknown, Unknown]]` in function `dict.__setitem__`
+ ::error file=openlibrary/plugins/openlibrary/lists.py,line=818,col=40,endLine=818,endColumn=88,title=Pyrefly unsupported-operation::Cannot set item in `dict[str, list[dict[Unknown, Unknown]]]`%0A  Argument `list[Thing | Unknown]` is not assignable to parameter `value` with type `list[dict[Unknown, Unknown]]` in function `dict.__setitem__`
+ ::error file=openlibrary/plugins/openlibrary/lists.py,line=823,col=42,endLine=825,endColumn=18,title=Pyrefly unsupported-operation::Cannot set item in `dict[str, list[dict[Unknown, Unknown]]]`%0A  Argument `list[Thing | Unknown]` is not assignable to parameter `value` with type `list[dict[Unknown, Unknown]]` in function `dict.__setitem__`
- ::error file=openlibrary/plugins/upstream/tests/test_merge_authors.py,line=37,col=9,endLine=37,endColumn=12,title=Pyrefly bad-override::Class member `MockSite.get` overrides parent class `Site` in an inconsistent manner%0A  `MockSite.get` has type `BoundMethod[MockSite, (self: MockSite, key: Unknown) -> Unknown]`, which is not assignable to `BoundMethod[MockSite, (self: MockSite, key: Unknown, revision: Unknown | None = None, lazy: bool | Unknown = False) -> Unknown | None]`, the type of `Site.get`
+ ::error file=openlibrary/plugins/upstream/tests/test_merge_authors.py,line=37,col=9,endLine=37,endColumn=12,title=Pyrefly bad-override::Class member `MockSite.get` overrides parent class `Site` in an inconsistent manner%0A  `MockSite.get` has type `BoundMethod[MockSite, (self: MockSite, key: Unknown) -> Thing | Unknown]`, which is not assignable to `BoundMethod[MockSite, (self: MockSite, key: Unknown, revision: Unknown | None = None, lazy: bool | Unknown = False) -> Thing | Unknown | None]`, the type of `Site.get`
+ ::error file=openlibrary/plugins/upstream/tests/test_merge_authors.py,line=142,col=12,endLine=142,endColumn=52,title=Pyrefly missing-attribute::Object of class `NoneType` has no attribute `key`