fix: Add option to skip target without srcs

[johan-vcc]

Description

Add option 'skip_if_no_srcs' to lint_clang_tidy aspect.
Set to True to skip target if it provides CcInfo but has no source files. By default this will generate an error.

See issue #406

Adding an option to skip these targets enables us to run ./lint.sh //package//... even if some target has no source files, e.g. code generation targets.

Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: yes
  Add option 'skip_if_no_srcs' to lint_clang_tidy aspect.

Test plan

• Manual testing; please provide instructions so we can reproduce:
  bazel build with lint_clang_tidy_aspect for a target that provides 'CcInfo' but does not have 'srcs' attribute.
  With skip_if_no_srcs set to True:
  Build completes with empty target.AspectRulesLintClangTidy.out file.
  Without new option / new option set to False:

File ".../aspect_rules_lint~/lint/clang_tidy.bzl", line 186, column 38, in _filter_srcs
                return [s for s in rule.files.srcs if _is_source(s)]
Error: No attribute 'srcs' in files. Make sure there is a label or label_list type attribute with this name

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