Say who is talking, when Projectile speaks unprompted

[bbatsov]

Projectile had five message conventions in one file: 12 sites said Projectile:,
3 said Projectile bare, 3 led with the project name, 11 passed a string
straight through, and 31 said nothing at all. The same event could come out two
ways - Invalidated Projectile cache for X from the command,
Projectile: invalidating the cache of X from a watch.

The manual, for the record, mandates no package prefix. It asks for no terminal
punctuation, suggests some-function: only "occasionally", and specifies the
Operating.../Operating...done idiom. Practice varies: project.el and
vc.el use no prefix, magit mostly doesn't, flycheck is mixed, and eglot -
in core - defines a helper that prefixes [eglot] . That last is the precedent
followed here.

One rule now, drawn along the line projectile-verbose already established:

• Unprompted -> [Projectile] , suppressed by projectile-verbose. Both
  come from one helper, which also retires fourteen hand-written
  (when projectile-verbose (message ...)) forms.
• A command answering for itself -> unprefixed. You know who is talking and
  the echo area is narrow.
• Asked for, but arriving later out of a sentinel or timer (the async
  indexer, the dashboard's index button) -> prefixed but never suppressed. By
  then the prefix is the only thing identifying the source.

Bracket form over Projectile: because it matches eglot, greps cleanly in
*Messages*, and stays legible when the message itself contains a colon.

Left alone deliberately: the three [project-name] messages, which say something
different and compose badly with a package prefix; the %s passthroughs in the
search/replace UIs, which have their own buffer for context; and the six
display-warning calls, which already render as Warning (projectile):.

Two manual conventions came along with it: terminal punctuation is gone from the
prefixed set, and the indexing notice now pairs Indexing X... with
Indexing X...done - it previously had a space before its ellipsis (which the
manual calls out) and never closed, so a long index left the echo area claiming
to still be working.