fix: use package name to build aggregate if present

[sofisl]

We've been having an ongoing bug, where release-please does not create a branch with the component name for the release (example: googleapis/gax-nodejs#1560)

This is weird, because the release-itself has the correct branch name up to this line:

release-please/src/manifest.ts

Line 749 in 72b0ab3

newReleasePullRequests.push({

But this correct name gets lost after the Merge plugin is run:

release-please/src/manifest.ts

Line 795 in 72b0ab3

return newReleasePullRequests.map(

The least disruptive solution I found is to mirror how the (correct) branch name gets selected in base.ts:

release-please/src/strategies/base.ts

Line 302 in 72b0ab3

const branchName = branchComponent

Base looks for a component or packageName to determine the type of branch. I think the merge strategy should do the same.

See a longer explanation of this bug (which I don't think this totally resolves, but at least preserves some functionality: #2214