Improve error message when using multiple sources for a package without markers

[zanieb]

e.g.,

[tool.uv.sources]
somepackage = [
    { path = "../somepackage", editable = true }
    { index = "pypi" }, 
]

Shows the error

error: Failed to parse: `pyproject.toml`
  Caused by: TOML parse error at line 81, column 9
   |
81 | somepackage = [
   |         ^
Source markers must be disjoint, but the following markers overlap: `true` and `true`.

hint: replace `true` with `python_version < '0'`.

We shouldn't be showing implicit true markers, and the hint is unhelpful.

Originally posted by @pirate in #14136 (comment)

[pirate]

do you know if theres a way to accomplish the desired behavior currently?

1. try to install package from local dir first (using an absolute local path outside the root, not a subdir/workspace)
2. fall back to pypi if local package or its pyproject.toml isnt found