UserArrayOption: Small cleanup in value parsing
UserArrayOption: Small cleanup in value parsing

It is nicer to early raise exception if the value from meson_options.txt
is not a string in "[]" format than duplicating the parser code for both
cases.

Also it was checking for duplicated items only in the user_input case,
but we should also check for dups in the default value from
meson_options.txt.

QuLogic/meson,MathieuDuponchelle/meson,thiblahute/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,pexip/meson,mesonbuild/meson,jeandet/meson,jeandet/meson,pexip/meson,MathieuDuponchelle/meson,jeandet/meson,pexip/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,MathieuDuponchelle/meson,thiblahute/meson,jpakkane/meson,becm/meson,thiblahute/meson,QuLogic/meson,jpakkane/meson,thiblahute/meson,QuLogic/meson,jpakkane/meson,thiblahute/meson,pexip/meson,becm/meson,becm/meson,QuLogic/meson,becm/meson,jeandet/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,becm/meson,pexip/meson,MathieuDuponchelle/meson,thiblahute/meson,MathieuDuponchelle/meson,jeandet/meson,mesonbuild/meson,becm/meson,jeandet/meson,becm/meson,thiblahute/meson,jpakkane/meson,jpakkane/meson,mesonbuild/meson,jeandet/meson,becm/meson,becm/meson,QuLogic/meson,MathieuDuponchelle/meson,pexip/meson,QuLogic/meson,jpakkane/meson,jeandet/meson,jeandet/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,pexip/meson,thiblahute/meson,jpakkane/meson,becm/meson,thiblahute/meson,jpakkane/meson,MathieuDuponchelle/meson,mesonbuild/meson