Rename package to ros_cross_compile

[emersonknapp]

• Update everything to call the package ros_cross_compile instead of cross_compile, before we release to PyPI
• Update the REAMDE instructions to reflect the name change, installation instructions, and invocation examples

Part of https://github.com/ros-tooling/cross_compile/issues/112

[codecov[bot]]

Codecov Report

Merging #125 into master will not change coverage. The diff coverage is 75%.

@@           Coverage Diff           @@
##           master     #125   +/-   ##
=======================================
  Coverage   76.92%   76.92%           
=======================================
  Files           3        3           
  Lines         221      221           
=======================================
  Hits          170      170           
  Misses         51       51

Flag	Coverage Δ
#unittests	76.92% <75%> (ø)	:arrow_up:

Impacted Files	Coverage Δ
ros_cross_compile/sysroot_compiler.py	80.79% <ø> (ø)
ros_cross_compile/__main__.py	0% <0%> (ø)
ros_cross_compile/ros_cross_compile.py	69.23% <100%> (ø)

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