Re-implement build_vignettes() from first principles

[hadley]

To remove GPL code so we can relicense

[hadley]

Commit that copied the code: c607d0f

The problem is actually copy_vignettes() and find_vignette_extras().

• copy_vignettes() comes from https://github.com/wch/r-source/blob/66fa35c1b7fe8993c1f4d8e25276d725238ff279/src/library/tools/R/build.R#L342-L357
• find_vignette_extras() comes from https://github.com/wch/r-source/blob/66fa35c1b7fe8993c1f4d8e25276d725238ff279/src/library/tools/R/build.R#L359-L373.