cargo_build_script now only prints warnings to stderr.

[UebelAndre]

Overview

This prevents noisy output from cargo_build_script from being printed to the terminal, instead two log files are generated (stdout, stderr) which contain the raw output of the underlying build.rs script

Details

Currently users see something similar to the following output when they use cargo_build_script:

(note, uplaoded as a file because there's a lot of output)
big_output.log

In general, this masks important information in a sea of arbitrary log lines. The changes in this PR update the output to:

INFO: From CargoBuildScriptRun external/cargo_raze__libssh2_sys__0_2_20/libssh2_sys_build_script.out_dir:
Build Script Warning: libssh2/src/bcrypt_pbkdf.c:84:46: warning: expression does not compute the number of elements in this array; element type is 'uint32_t' (aka 'unsigned int'), not 'uint64_t' (aka 'unsigned long long') [-Wsizeof-array-div]
Build Script Warning:         blf_enc(&state, cdata, sizeof(cdata) / sizeof(uint64_t));
Build Script Warning:                                       ~~~~~  ^
Build Script Warning: libssh2/src/bcrypt_pbkdf.c:65:14: note: array 'cdata' declared here
Build Script Warning:     uint32_t cdata[BCRYPT_BLOCKS];
Build Script Warning:              ^
Build Script Warning: libssh2/src/bcrypt_pbkdf.c:84:46: note: place parentheses around the 'sizeof(uint64_t)' expression to silence this warning
Build Script Warning:         blf_enc(&state, cdata, sizeof(cdata) / sizeof(uint64_t));
Build Script Warning:                                              ^
Build Script Warning: 1 warning generated.

And log files are generated for the raw stderr and stdout from the build script, ie:

bazel-out/darwin-fastbuild/bin/external/cargo_raze__libssh2_sys__0_2_20/libssh2_sys_build_script.stderr.log
bazel-out/darwin-fastbuild/bin/external/cargo_raze__libssh2_sys__0_2_20/libssh2_sys_build_script.stdout.log

(note, uplaoded as a file because there's a lot of output)
libssh2_sys_build_script.stderr.log

Additionally, the deps attribute of cargo_build_script now only accepts targets that return the DepInfo provider. All other dependencies should be passed via data.

[UebelAndre]

@damienmg @illicitonion hey, do either of you have bandwidth for a review here? 🙏