Sanitize stray newlines at point of origin

Mentioned in the comments: A user can add a stray newline to an input and mess up the formatting. The original implementation guarded against this at the point of origination. The basic concept is that user input isn't to be trusted, and at the point that they've given it to us, we can sanitize it for consistency https://github.com/heroku/buildpacks-ruby/blob/dda4ede413fc3fe4d6d2f2f63f039c7c1e5cc5fd/commons/src/output/build_log.rs#L224. With that concept in mind, the BuildOutput needs to own the consistency of the data it passes to other modules and functions.

The build output concept largely takes ownership over newlines between outputs so I feel comfortable trimming these. I picked shared entry points to trim before any writeln_now calls. 

I extended the test to check for trailing newlines in additional locations.

libherokubuildpack/src/buildpack_output/mod.rs

@@ -270,6 +270,7 @@ where
 
     fn write_paragraph(&mut self, color: &ANSI, s: impl AsRef<str>) {
         let io = self.state.write_mut();
+        let contents = s.as_ref().trim();
 
         if !io.was_paragraph {
             writeln_now(io, "");
@@ -279,7 +280,7 @@ where
             io,
             ansi_escape::wrap_ansi_escape_each_line(
                 color,
-                prefix_lines(s.as_ref(), |_, line| {
+                prefix_lines(contents, |_, line| {
                     // Avoid adding trailing whitespace to the line, if there was none already.
                     // The `\n` case is required since `prefix_lines` uses `str::split_inclusive`,
                     // which preserves any trailing newline characters if present.
@@ -330,7 +331,7 @@ where
             &mut self.state.write,
             ansi_escape::wrap_ansi_escape_each_line(
                 &ANSI::BoldPurple,
-                format!("\n# {}\n", buildpack_name.as_ref()),
+                format!("\n# {}\n", buildpack_name.as_ref().trim()),
             ),
         );
 
@@ -357,7 +358,7 @@ where
     const PREFIX_REST: &'static str = "  ";
 
     fn style(s: impl AsRef<str>) -> String {
-        prefix_first_rest_lines(Self::PREFIX_FIRST, Self::PREFIX_REST, s.as_ref())
+        prefix_first_rest_lines(Self::PREFIX_FIRST, Self::PREFIX_REST, s.as_ref().trim())
     }
 
     /// Begin a new section of the buildpack output.
@@ -406,7 +407,7 @@ where
     const CMD_INDENT: &'static str = "      ";
 
     fn style(s: impl AsRef<str>) -> String {
-        prefix_first_rest_lines(Self::PREFIX_FIRST, Self::PREFIX_REST, s.as_ref())
+        prefix_first_rest_lines(Self::PREFIX_FIRST, Self::PREFIX_REST, s.as_ref().trim())
     }
 
     /// Emit a step in the buildpack output within a section.
@@ -538,8 +539,11 @@ mod test {
     #[test]
     fn write_paragraph_empty_lines() {
         let io = BuildpackOutput::new(Vec::new())
-            .start("Example Buildpack")
+            .start("Example Buildpack\n\n")
             .warning("hello\n\n\t\t\nworld")
+            .section("Version\n\n")
+            .step("Installing\n\n")
+            .finish()
             .finish();
 
         let tab_char = '\t';
@@ -552,6 +556,8 @@ mod test {
             ! {tab_char}{tab_char}
             ! world
 
+            - Version
+              - Installing
             - Done (finished in < 0.1s)
         "};