The Zip archives emitted by `rust_doc` rules used to contain a long
`/bazel-bin/k8-fastbuild/path/to/target` prefix before each archive
entry name. This made it hard to work with the generated archive. This
patch adds a transformation to

This could be written in Starlark using the `DirectoryExapnder` API, but
that was only introduced in Bazel 3.4.0. Since we have a minimum Bazel
version of 0.17.1, we instead write the helper as a small Python script.

Fixes bazelbuild#471.

Test Plan:
Unit tests included. As an end-to-end test, run

```
cd examples/ &&
bazel build //hello_world:hello_world_doc &&
unzip -l bazel-bin/hello_world/hello_world_doc.zip
```

to list the contents of the built `hello_world_doc.zip` archive. Before
this patch, the result was like:

```
Archive:  bazel-bin/hello_world/hello_world_doc.zip
  Length      Date    Time    Name
---------  ---------- -----   ----
        0  2010-01-01 00:00   bazel-out/k8-fastbuild/bin/hello_world/hello_world_doc/.lock
     1792  2010-01-01 00:00   bazel-out/k8-fastbuild/bin/hello_world/hello_world_doc/COPYRIGHT.txt
     4421  2010-01-01 00:00   bazel-out/k8-fastbuild/bin/hello_world/hello_world_doc/FiraSans-LICENSE.txt
...
     2132  2010-01-01 00:00   bazel-out/k8-fastbuild/bin/hello_world/hello_world_doc/storage.js
     1180  2010-01-01 00:00   bazel-out/k8-fastbuild/bin/hello_world/hello_world_doc/theme.js
     3764  2010-01-01 00:00   bazel-out/k8-fastbuild/bin/hello_world/hello_world_doc/wheel.svg
---------                     -------
   890555                     38 files
```

After this patch:

```
Archive:  bazel-bin/hello_world/hello_world_doc.zip
  Length      Date    Time    Name
---------  ---------- -----   ----
        0  2010-01-01 00:00   .lock
     1792  2010-01-01 00:00   COPYRIGHT.txt
     4421  2010-01-01 00:00   FiraSans-LICENSE.txt
...
     2132  2010-01-01 00:00   storage.js
     1180  2010-01-01 00:00   theme.js
     3764  2010-01-01 00:00   wheel.svg
---------                     -------
   890555                     38 files
```

wchargin-branch: rustdoc-strip-prefix
wchargin-source: 29978cf3bdb7de6cee154f8c11d0251574c2b104

wchargin-branch: rustdoc-strip-prefix
wchargin-source: 69b2e78f1f52c88609aff631c908471c7521c5f7

wchargin-branch: rustdoc-strip-prefix
wchargin-source: 40227180c7f910d9f76716cf42621e08e5f808a1

wchargin-branch: rustdoc-strip-prefix
wchargin-source: 8c46290195fc0668f3126589fd07ee863df8749f

wchargin-branch: rustdoc-strip-prefix
wchargin-source: 838287ca34eb69c27f62e4a0900dbb7ac6a3ebd0

wchargin-branch: rustdoc-strip-prefix
wchargin-source: ea88a0484df0608cb08521fb4cde2d4dc7904553

wchargin-branch: rustdoc-strip-prefix
wchargin-source: ea88a0484df0608cb08521fb4cde2d4dc7904553

wchargin-branch: rustdoc-strip-prefix
wchargin-source: aba05e35e129db74d032bcbfdb7764233a2d94ce