Resolve dependencies when imports are relative to the package path

[yushan26]

When # gazelle:python_generation_mode package is enabled, relative imports are currently not being added to the deps field of the generated target.

For example, given the following Python code:

from .library import add as _add
from .library import divide as _divide
from .library import multiply as _multiply
from .library import subtract as _subtract

The expected py_library rule should include a dependency on the local library package:

py_library(
    name = "py_default_library",
    srcs = ["__init__.py"],
    visibility = ["//visibility:public"],
    deps = [
        "//example/library:py_default_library",
    ],
)

However, the actual generated rule is missing the deps entry:

py_library(
    name = "py_default_library",
    srcs = ["__init__.py"],
    visibility = ["//visibility:public"],
)

This change updates file_parser.go to ensure that relative imports (those starting with a .) are parsed and preserved. In Resolve(), logic is added to correctly interpret relative paths:

A single dot (.) refers to the current package.

Multiple dots (.., ..., etc.) traverse up parent directories.

The relative import is resolved against the current label.Pkg path that imports the module and converted into an path relative to the root before dependency resolution.

As a result, dependencies for relative imports are now correctly added to the deps field in package generation mode.

[linzhp]

Good job in covering package mode. Can we also have test cases covering file and project mode?

It seems "testdata/relative_imports" and "testdata/reslove_deps_relative_imports" are covering similar things. From the name, it's also hard to tell the difference between "testdata/relative_imports" and "testdata/reslove_deps_relative_imports"

We can either:

1. have all test cases in "testdata/relative_imports", with some packages in package mode, some in project mode and some in file mode, or
2. have "testdata/relative_imports_package_mode", "testdata/relative_imports_file_mode" and "testdata/relative_imports_project_mode"

[yushan26]

I think 2 is preferrable? Seems more straight forward and the behavior would not change in project mode or file mode.
Is file mode also something that we want to look into supporting too? I think if we do, then implementations for relative paths in file mode should be in a separate PR.

[linzhp]

We can support file mode in the next PR.