Use builtin . instead of .

[nkakouros]

Due diligence

• I am familiar with the "Reporting issues" section of CONTRIBUTING.md
• I have searched the existing issues and haven't found one that matches what I'm reporting or requesting now.
• If this is a bug report, I have searched the Bash changelog for information that may explain or provide clues to the behavior I'm observing and reference it in the body of the report.

Description

_GO_USE_MODULES uses . to source the library files. It might be the case that go-script is used together with other frameworks that shadow the builtin . with eg an alias. In that case, go-script will use the alias with unpredicted results. I think it would be nice if instead of ., builtin . was used.