[Bazel] Make td_library usable as data

This patch makes it possible to list a td_library as a rule's data
attribute and get its source files and all its transitive dependencies
at runtime. This is useful for, e.g. shell tests running tblgen.

Note that this is a bit different from how a "normal" (e.g. C++) library
rule would work because those have actual library outputs and the
td_library rule just bundles some source files and includes. If someone
wanted to make use of the includes, they would have to access the TdInfo
provider, but this keeps simple things simple.

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D106922