The URLs are in the .mk files
e.g. https://github.com/JuliaLang/julia/blob/release-1.4/deps/pcre.mk
https://github.com/JuliaLang/julia/blob/release-1.4/deps/blas.mk
and those third parties have changed the location so they are no longer present at those addresses
You will have to hunt them down manually and copy them to srccache yourself (or fix the .mk files and submit a PR)