fix(gazelle): Do not resolve absolute imports to sibling modules (#3106)

Currently, gazelle allows absolute imports to be resolved to sibling
modules: an `import foo` statement will resolve to a `foo.py` file in
the same folder if such file exists. This seems to be a Python 2
behavior (ie. pre-`from __future__ import absolute_import`), and doesn't
work on the current rules_python setup.

This behavior is explicitly tested in the
[siblings_import](https://github.com/bazel-contrib/rules_python/tree/cbe6d38d01c14de46d90ea717d0f2090117533fa/gazelle/python/testdata/sibling_imports)
test case. However, recreating the exact same repository layout from
this test case and running `bazel test //pkg:unit_test`, the test fails
with the import failing.

This PR adds a new directive, `gazelle:python_resolve_sibling_imports`,
to allow disabling such behavior.

The actual changes are in 3 places:

- In `gazelle/python/target.go`, the directive is added to `if
t.siblingSrcs.Contains(fileName) && fileName !=
filepath.Base(dep.Filepath)`, which is where the import is converted to
a full absolute import if it matches a sibling file;
- In `gazelle/python/generate.go`, the handling of `conftest.py` was
dependent on this behavior (ie. it added a dependency on the module
`conftest`, assuming that it would be resolved to the relative module).
That was modified to compute the full absolute module path instead.
- In `gazelle/python/resolve.go`, resolve relative imports even when
using file generation mode.

I also explicitly added `gazelle:python_resolve_sibling_imports true` to
any test that breaks if the default value of this directive is changed
to `false`.