Ignore dotfiles in lookup_srcs makefile function

[jerryz123]

This makes the find and fd cases match, as fd already ignores dotfiles.
The primary use case is to ignore Emacs lockfiles of the form .#SourceFile.scala.

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

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• Did you set main as the base branch?
• Is this PR's title suitable for inclusion in the changelog and have you added a changelog:<topic> label?
• Did you state the type-of-change/impact?
• Did you delete any extraneous prints/debugging code?
• Did you mark the PR with a changelog: label?
• (If applicable) Did you add documentation for the feature?
• (If applicable) Did you add a test demonstrating the PR?

• (If applicable) Did you mark the PR as Please Backport?

[jerryz123]

Tests are unrelated, will be fixed by #1163

[abejgonzalez]

LGTM