Skip install step when a package has nothing to install

[luca-della-vedova]

As mentioned in #39 (comment). Originally authored in luca-della-vedova#2 by @mxgrey, I only contributed unit tests to make sure the functionality works as intended and no regressions are added unintentionally.

Original PR description:

While testing out colcon test for colcon-cargo, I found that we get a build error from colcon build if there's a Rust package that does not contain any binary crates (i.e. it only contains a library crate).

This PR avoids that problem by checking each cargo package before running cargo install to ensure it contains at least one binary crate. If there are no binary crates in the package then we simply skip the cargo install step to avoid the spurious error code.

With this, we can successfully use colcon build followed by colcon test to run tests for library-only cargo packages.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 76.47059% with 4 lines in your changes missing coverage. Please review.

Project coverage is 66.96%. Comparing base (48cd670) to head (18ad6b5).

Files	Patch %	Lines
colcon_cargo/task/cargo/build.py	76.47%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
+ Coverage   66.02%   66.96%   +0.93%     
==========================================
  Files           6        6              
  Lines         209      224      +15     
  Branches       30       36       +6     
==========================================
+ Hits          138      150      +12     
- Misses         50       52       +2     
- Partials       21       22       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[luca-della-vedova]

The functionality is quite uncontroversial and with good coverage, but I'll leave the PR open for some time in case some other members of the community want to pitch in @maspe36, @esteve @jhdcs