Show sources in rye show and support inline tables

[mitsuhiko]

Add support for inline tables for sources. Now the following two formats are equivalent:

[[tool.rye.sources]]
name = "test"
url = "https://test.pypi.org/simple/)"

[tool.rye]
sources = [
  { name = "test", url = "https://test.pypi.org/simple/)" }
]

Additionally rye show now spits out which sources are configured:

$ rye show
project: x
...
configured sources:
  default (index: https://pypi.org/simple/)
  test (index: https://test.pypi.org/simple/)

Fixes #485