[ethPM] Add expression begin/close chars to package name regex

[njgheorghita]

What was wrong?

Package name regex expression needs expression beginning/closing chars (^/$) to correctly validate package names.

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[pipermerriam]

Strikes me as something that could have a test added to validate whatever case this fixes.

[njgheorghita]

@pipermerriam Definitely, a little suspicious about how it went this long without getting a test but it's there now.

[pipermerriam]

👍

(if you didn't, you should verify that the expected tests do indeed fail without this patch before merging to be extra sure everything is behaving as expected)