feat: preserve existing data in [project]

[mkniewallner]

Closes #52.

Preserve existing data in [project] section of pyproject.toml when performing the migration. This can be opted out by setting --replace-project-section flag.

This should also enable support for projects using PEP 621 with Poetry, but official support and tests will be added in another PR.

There are some edge cases that are not handled, but that are I believe not worth handling.

For instance, if a project using Pipenv has the following Pipfile:

[[source]]
url = "https://pypi.org/simple"
verify_ssl = true
name = "pypi"

[[source]]
url = "https://example.com/simple"
verify_ssl = true
name = "other-index"

[packages]
dep = "==1.2.3"
private-dep = { version = "==1.2.3", index = "other-index" }
local-package = { path = "package/" }
git-dep = { git = "https://example.com/foo/bar.git", ref = "v1.2.3" }

and the following pyproject.toml:

[project]
name = "for"
version = "1.0.0"
requires-python = ">=3.13"
dependencies = ["bar==1.2.3"]

The project would get migrated to:

[project]
name = "foo"
version = "1.0.0"
requires-python = ">=3.13"
dependencies = ["bar==1.2.3"]

[tool.uv]
package = false

[[tool.uv.index]]
name = "pypi"
url = "https://pypi.org/simple"

[[tool.uv.index]]
name = "other-index"
url = "https://example.com/simple"
explicit = true

[tool.uv.sources]
private-dep = { index = "other-index" }
local-package = { path = "package/" }
git-dep = { git = "https://example.com/foo/bar.git", rev = "v1.2.3" }

where [tool.uv.sources] would reference dependencies that do not exist, since we keep the existing dependencies key.

We could conditionally add sources only if dependencies is replaced, or consider an option to merge some sections, but I feel like this would make things way more complex, for something that I believe is not really a use case to begin with.