Parsing source files on excluded files

[csgillespie]

When I call lintr::lint on a file, it runs get_source_expression regardless if the file should be excluded

I think if we move the get_source_expression down a few lines, this should solve the issue.

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The use case here is I run lintr::lint on a bunch of .Rmd files. But occasionally I want to exclude the files from linting.