taskdata: Improve handling of regex in ASSUME_PROVIDED
ASSUME_PROVIDED can take regexs however the current way of handling
this in code is suboptimal. It means that you can add something like:
DEPENDS += "texinfo-nativejunk-that-does-not-exist"
and if texinfo-native is in ASSUME_PROVIDED, no error will occur.
Update the code to only treat something as a regex if a start or end
anchor character is present (which wouldn't be valid in a recipe name).
[YOCTO #13893]
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>