runfiles: Fix usage instructions

[fmeum]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Docs reference bazel_tools and import instructions fail with Bzlmod.

What is the new behavior?

The canonical location for the runfiles library going forward will be the rules_python repo, so users should load from it. Since repository names are essentially dynamic with Bzlmod, they should not be used in import statements.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[fmeum]

CC @alexeagle